Methylammonium halide salt interfacial modi�cation of perovskite quantum dots/triple-cation perovskites enable e�cient solar cells

Perovskite solar cells (PeSCs) have been introduced as a new photovoltaic device due to their excellent power conversion e�ciency and low cost. However, due to the limitations of the perovskite �lm itself, the existence of defects was inevitable, which seriously affects the number and mobility of carriers in perovskite solar cells, thus restricting PeSCs Improved e�ciency and stability. Here, we use methylammonium halide salts (MAX, X = Cl, Br, I) to modify the interface between perovskite quantum dots (PeQDs) �lm and triple-cation perovskite �lm, which can repair the surface defects of PeQDs �lm, thereby improving the crystal quality of triple-cation perovskite �lm. Ultimately, we achieved high short-circuit current density, high open-circuit voltage, and 20.4% power conversion e�ciency in PeQDs/triple-cation PeSCs.


I. Introduction
Due to the many excellent material properties exhibited by organic-inorganic halide perovskite materials, such as high extinction coe cient, high carrier mobility, and micron-scale carrier diffusion length [1][2][3][4][5].Perovskite solar cells (PeSCs) have attracted extensive attention in the scienti c research community in the past decade and are considered to be very promising photovoltaic materials [6,7].Its power conversion e ciency (PCE) also rose from 3.8% to 25.7% in just a few years [8].However, due to the low preparation temperature of perovskite materials and the di culty in controlling the crystallization process, it was easy to cause a large number of defects on the surface and grain boundaries of the nal perovskite lm [9][10], including uncoordinated Pb 2+ , iodine vacancies, Iodine interstitial atoms, lead vacancies and lead-iodine transposition defects, etc.These defects often cause nonradiative recombination and ion migration of carriers, thereby reducing the PCE and long-term stability of the PeSCs [11].
At present, additives engineering [12] and interface engineering [13] were the major methods to reduce defects in passivation perovskite lms.In particular, additives engineering can control the crystallization process and passivate defects by introducing passivation substances into the precursor solution, which has the advantages of simple operation and remarkable effect.In the process of realizing the defect passivation, the functional group of the passivator molecule was very important.Carbonyl group [14], amino group [15], carboxyl group [16] and phenethylammonium iodide [17] passivate defects by forming coordination bonds with unsaturated dangling bonds, thereby prolonging the carrier lifetime and improving device performance parameters.To date, a variety of molecules with different functional groups have been introduced into perovskite precursors as passivating agents.For example, Wang et al.
[18] introduced caffeine into the perovskite precursor, used the strong interaction between caffeine's C=O and Pb 2+ to increase the nucleation activation energy, thereby delaying the perovskite nucleation rate and increasing the perovskites quality, the nal device obtains 20.25% PCE.Chen et al. [19] synthesized a πconjugated and alcohol-soluble small molecule with bilateral carboxyl and thiophene groups, namely 2,5di(thiophen-2-yl)terephthalic acid (DTA), and added it to the ammonium salt precursor to prepare the perovskite lm uses its electron-rich carboxyl groups to form Lewis acid-base adducts with uncoordinated Pb 2+ in the perovskite lm to passivate grain boundaries and surface defects, and nally reduce the device voltage loss to 0.38 V.Although these reported passivator molecules show obvious passivation effect on defects in perovskite lms, there were also problems such as complex molecular structure and di cult synthesis.In addition, some surface passivators need to use benzene substances that are harmful to the environment as solvents when they were used for surface treatment of perovskite lms, which was not conducive to environmental protection and human health [20].Therefore, it was still of great signi cance to nd novel passivators with simple structure and environmental friendliness as additives to be introduced into perovskites to prepare high-quality perovskite lms and high-performance PeSCs.This work reports an effective passivator for resolving perovskite surface defects, namely methylammonium halide salt, to modify the interface between perovskite quantum dots (PeQDs) lm and Cs 0.05 FA 0.81 MA 0.14 PbBr 0.14 I 2.86 (CsFAMA) triple-cation perovskite lm, reducing defects in perovskites.In addition, the use of PeQDs lm in the underlying layer of the triple-cation perovskite lm can increase the light utilization rate and short-circuit current, thereby improving the PeQDs/triple-cation PeSCs.performance.

Synthesis of CsPbI 3 perovskite quantum dots
Cs-oleate precursor solution was synthesized by Cs 2 CO 3 (0.407 g), ODE (20 mL), and OA (1.25 mL) in a 45 mL ask at 120 °C for 30 min under stirring.PbI 2 (0.5 g) and ODE (25 mL) were stirred in a 4 -mL ask at 120 °C for 30 min.Add preheated (130 °C) OA (2.5 mL) and OAm (2.5 mL) to the PbI 2 -ODE reaction ask until the PbI 2 was completely dissolved.Then, 2 mL of the Cs-oleate precursor was swiftly injected into the PbI 2 reaction mixture at 180 °C and then the CsPbI 3 mixture was quenched into an ice bath.To purify the QDs, EA solution was added to the CsPbI 3 crude solution with 3:1 in volume ratio and then centrifuged at 6000 rpm for 5 min.The bottom QDs precipitate was added to hexane and EA (1:1 in volume ratio), sonicated for 5 min, and then centrifuged at 6000 rpm for 5 min.Finally, the obtained CsPbI 3 QDs precipitate was dispersed in 3 mL of octane, then store refrigerated for at least 24 hours, and use the supernatant via centrifugation at 6000 rpm for 5 min.

Device Fabrication
Patterned FTO anodes were sequentially cleaned by de-ionized water, acetone, ethanol, and isopropanol in an ultrasonic cleaner, and a following ultraviolet ozone treatment for 15 min. of nickel nitrate (0.291 g) was dissolved in ethylenediamine (0.067 mL) and ethylene glycol (1 mL) with magnetic stirring at room temperature for 24 h.The NiOx hole transporting layer was deposited by spin-coating the filtered NiOx precursor solution on an FTO substrate at 3000 rpm for 30 s and annealed on a hot plate at 120 °C for 10 min, followed by baking in an oven at 300 o C for 1 h.60 μL of CsPbI 3 PeQDs solution was uniformly distributed on the NiOx hole transporting layer at 1000 rpm for 20 s. 5 mg of methylammonium halide salts (MAX, X = Cl, Br, I) and 5 mL of ethyl acetate were mixed and sonicated for 20 min, and then centrifuged at 6000 rpm for 5 min to obtain the MAX (X = Cl, Br, I) salt solution.Next, 100 μL of MAX salt solution was spin-coated on the CsPbI 3 PeQDs layer at 2000 rpm for 10 s.Dissolve 461 mg of PbI 2 , 139 mg of FAI, 12.8 mg of CsI, and 15.7 mg of MABr powders in 1 mL of dimethyl sulfoxide (DMSO)/dimethylformamide (DMF) (1/4 vol/vol) at room temperature 25°C for about 22 h, to prepare the Cs 0.05 FA 0.81 MA 0.14 PbBr 0.14 I 2.86 triple-cation perovskite precursor solution at a concentration of 1 M. 80 µL of perovskite precursor solution was spin-coated on the PeQDs/MAX layer by a two-step method, using 1000 rpm for 10 s and 5000 rpm for 40 s, respectively.Quickly drop 100 μL of toluene antisolvent onto the PeQDs/MAX layer during the remaining 20 s of the second stage.After the rotation, it was placed in a petri dish for 5 min, and then placed on a hot plate at 100 °C for 10 min to form a Cs 0.05 FA 0.81 MA 0.14 PbBr 0.14 I 2.86 triple-cation perovskite lm.Subsequently, a 20-nm-thick C 60 electron transport layer and a 5-nm-thick BCP electron blocking layer were sequentially deposited by evaporation under high vacuum.Finally, a 100 nm-thick Ag top electrode was deposited, and its PeQDs/MAX/CsFAMA PeSC structure schematic and cross-sectional SEM image were shown in Figure 1.

Characterization
Absorption spectra of perovskite lms were tested using a V-770 UV/VIS/NIR spectrophotometer (Jasco, Japan).Use a uorescence spectrophotometer model Hitachi F-7000 (Tokyo, Japan) to test the photoluminescence (PL) spectrum of perovskite lms.X-ray diffraction (XRD) patterns of perovskite lms were tested using a PANalytical X'Pert PRO MRD diffractometer (Almelo, The Netherlands) using CuKα (λ = 1.5418Å) radiation source.The surface morphologies and high-resolution transmission electron microscopy (TEM) image of the perovskite lms and PeQDs were observed using a ZEISS Sigma eld emission scanning electron microscope (FESEM) instrument (ZEISS, Germany) and a JEM-2100F transmission electron microscope instrument (JEOL, Japan), respectively.The photocurrent-voltage (J-V) curves of the PeQDs/triple-cation PeSCs were obtained by using a MFS-PV-Basic solar simulator (Hong-Ming Technology Co., Ltd., Taiwan) with a Keithley 2420 source meter under illumination of AM 1.5G simulated sunlight at 100 mW cm -2 , calibrated by a NREL PVM-894 standard silicon reference cell (PV Measurements Inc., USA).External quantum e ciency (EQE) was tested by LSQE-R QE system (LiveStrong Optoelectronics Co., Ltd., Taiwan) equipped with a LAMBDA 35 UV-VIS-NIR spectrophotometer (PerkinElmer, USA) and a XES-204S 150 W xenon lamp (San-Ei Electric Co., Ltd., Japan) as a light source.

Iii. Results And Discussion
The optical properties and morphology of CsPbI 3 PeQDs prepared by hot-injection method were characterized.In the absorption spectrum of PeQDs solution in Fig. 2a, it can be found that its absorption peak was about 715 nm, and the corresponding uorescence spectrum (PL) measured the PL peak also at 715 nm. Figure 2b depicts the XRD pattern of CsPbI 3 PeQDs lm, indicating that black α-CsPbI 3 perovskite diffraction peaks appear at 14.33°, 20.23°, and 28.86°, which correspond to the (100), (110), and (200) planes.It was evident from the high-resolution TEM image in Fig. 2c that the as-synthesized CsPbI 3 PeQDs were in cubic phase with an average size of about 18.6 nm (Fig. 2d).
In order to explore the effect of the introduction of MAX (X = Cl, Br, I) salts on the surface morphology of PeQDs/CsFAMA perovskite lms, the above perovskite lms were characterized by SEM, as shown in Fig. 3. Figure 3a shows the pure CsFAMA perovskite lm, it can be seen that the surface particle size was small and prone to defects and pores.When the CsFAMA perovskite lm was covered on the PeQDs lm, the particle size of the CsFAMA perovskite becomes larger and the grain boundaries were slightly reduced, as shown in Fig. 3b.Since grain boundaries were one of the main locations for defects, perovskite lms with larger grains have fewer grain boundaries, thereby obtaining higher lm quality.From Figs. 3c-3e, between the introduction of MAX (X = Cl, Br, I) salts to PeQDs/CsFAMA perovskite, it can be observed that the particle size of CsFAMA perovskite increases signi cantly.The reason for the enlarged perovskite grains may be due to the e cient modi cation of the PeQDs layer by MAX (X = Cl, Br, I) salts.When CsFAMA perovskites were coated on PeQDs/MAX (X = Cl, Br, I), the rapid nucleation of perovskite may be inhibited and perovskites tend to grow into larger-sized grains at low nucleation density.The results show that the PeQDs/MAI/CsFAMA perovskite has fewer grain boundaries, which can effectively reduce defects.The smooth surface of the perovskite lm was bene cial to improve the interface contact between the perovskite layer and the hole transport layer, and improve the photo-generated charge transfer e ciency [21].
Figure 4 shows the X-ray diffraction patterns of CsFAMA, PeQDs/CsFAMA, and PeQDs/MAX (Cl, Br, I)/CsFAMA lms, in which the XRD patterns with stronger peaks can represent the formation of crystalline CSFAMA lms.The main XRD diffraction peak intensity of the PeQDs/CsFAMA lm was higher than that of the pure CsFAMA perovskite lm, which indicates that the CsFAMA coating on the PeQDs effectively reduces defects.When the MAX (Cl, Br, I) salts were further introduced into PeQDs/CsFAMA, the XRD intensities of the diffraction peaks of the PeQDs/MAI/CsFAMA lms were higher than those of the other two (PeQDs/MACl/CsFAMA and PeQDs/MABr/CsFAMA), representing better lm quality.In addition, the PbI 2 peak (001) appeared at 12.7°, it can be found that the PbI 2 peak intensities of the PeQDs/MAI/CsFAMA and PeQDs/MABr/CsFAMA lms were lower, indicating that the formation of PbI 2 was reduced, which can make it easier for carriers to transition to the hole transport layer, and will not be blocked to cause recombination between carriers.
Figure 5a shows the PL spectra of CsFAMA, PeQDs/CsFAMA, and PeQDs/MAX (Cl, Br, I)/CsFAMA lms measured under the excitation of wavelength 405 nm.The PL peak of the CsFAMA lm was at 790 nm, while the PeQDs/CsFAMA and PeQDs/MAX (Cl, Br, I)/CsFAMA lms have a slight blue shift compared to the CsFAMA lm.The blue shift may be caused by the disappearance of shallow defect energy level after surface defects were passivated, resulting in an increase in the energy band width [22].In addition, the defect level accelerates the non-radiative recombination in the perovskite lm, resulting in a decrease in the carrier concentration of the perovskite lm after reaching steady-state equilibrium under photoexcitation conditions, thereby reducing the radiative recombination rate, i.e., the PL intensity decline.This can also be seen from the intensity of the PL peak.The PL intensity of the CsFAMA lm was signi cantly improved after passivation treatment with PeQDs and MAX (Cl, Br, I) salts.Among them, the PeQDs/MAI/CsFAMA lm was the most effective in reducing defects, which proves that the non-radiative recombination in the lm was suppressed.
The absorption spectra of CsFAMA, PeQDs/CsFAMA, and PeQDs/MAX (Cl, Br, I)/CsFAMA lms were shown in Fig. 5b.Compared with the CsFAMA perovskite lm, the absorption edge of the perovskite lm after passivation with PeQDs and MAX (Cl, Br, I) salts did not change signi cantly, indicating that they did not affect the composition and energy gap of the CsFAMA perovskite lm.However, the PeQDs/CsFAMA lms had better light absorption after the introduction of MAX (Cl, Br, I) salts, which may be due to the improved crystalline quality of the perovskite lms.
To further explore the effect of introducing MAX (X = Cl, Br, I) salts on the photovoltaic performance of PeSCs, and to evaluate the reproducibility of PeSCs, more than 30 cells were fabricated and tested in each case.Figure 6 shows the statistical results of photovoltaic parameters of the PeSCs based on the CsFAMA, PeQDs/CsFAMA, and PeQDs/MAX (Cl, Br, I)/CsFAMA.The device of pristine CSFAMA PeSC had an averaged short-circuit current density (Jsc) of 23.1 mA cm − 2 , an open-circuit voltage (Voc) of 0.93 V, a ll factor (FF) of 76.9%, and a PCE of 16.2%.After treating by MAI salt, the parameters were enhanced to an averaged Jsc of 24.6 mA cm − 2 , a Voc of 1.03 V, an FF of 77.8%, and a PCE of 19.6%.The average photovoltaic properties of all other PeSCs were also better than the pristine CSFAMA PeSC, indicating the reliability of the testing results.This indicates that the introduction of MAX (X = Cl, Br, I) salts were believed to play an important role in improving photovoltaic parameters.Figure 7 shows the J-V curves of the best-performing PeSCs in each case, and the corresponding photovoltaic performance parameters were summarized in Table 1.The PeSC based on pristine CsFAMA gave a PCE of 16.6% with a Jsc of 23 mA cm − 2 , a Voc of 0.94 V, and a FF of 7.1%.When the PeQDs layer was introduced into the CsFAMA PeSC, its Jsc, Voc, FF and PCE were signi cantly increased to 24.2 mA/cm − 2 , 0.977 V, 76.7% and 18.1%, which indicated that adding the PeQDs layer could improve the quality of the CsFAMA lm and at the same time increase the photocurrent.When MAX (X = Cl, Br, I) salts modi ed the PeQDs/CsMAFA interface, the optimized PeSC fabricated with MAI-treated PeQDs/CsMAFA perovskite exhibited an increased Jsc of 24.6 mA cm − 2 , a Voc of 1.04 V, an FF of 79.9%, subsequently, an enhanced PCE of 20.4%.Additionally, the devices based on MACl-and MABr-treated PeQDs/CsMAFA interface just gave the PCEs of 18.8% and 19.5%, respectively, validating the positive effects of this surface passivation process.The positive effect of this surface passivation process to effectively reduce defects was veri ed by modifying the PeQDs/CsMAFA interface with MAX (X = Cl, Br, I) salts.Figure 8a shows the EQE spectra of the PeSCs based on the CsFAMA, PeQDs/CsFAMA, and PeQDs/MAX (Cl, Br, I)/CsFAMA.It can be seen that adding a PeQDs layer to CsFAMA PeSC can effectively increase its Jsc and EQE, and there was a relatively obvious increase in the band of 500-750 nm.Through the quantum con nement effect, the absorption generated by PeQDs occurs in this band.Furthermore, the modi cation/passivation of the interface between PeQDs and CsFAMA using MAX (X = Cl, Br, I) salts further improve the lm crystallinity and reduce defects.The quantum e ciency of the display device in this wavelength range is greatly affected by the interface behind the light-absorbing layer.This phenomenon shows that the carrier recombination at this interface is signi cantly suppressed after passivation, and it also explains the improvement mechanism of the short-circuit current of the device by passivation.To investigate the effect of MAX (X = Cl, Br, I) salts on the stability of PeSCs, the PCE decays of MAX salt-treated PeSCs and control CsFAMA PeSCs were tracked and recorded at 25°C in nitrogen storage and under AM1.5G illumination.The PCE loss of CsFAMA PeSC was more than 80% after 120 h storage.In addition, the addition of the PeQDs layer to PeQDs/CsFAMA PeSC effectively reduced defects and greatly improved the phenomenon of PCE attenuation.The PeSCs treated with MAX (X = Cl, Br, I) salts retained 78.7%, 85.1% and 77.9% of their initial e ciencies, respectively (Fig. 8b).On the other hand, the reason for the more attenuation of PeQDs/MAI/CsFAMA PeSCs were that the bonding between iodide ions and organic cations were weak, which leads to easy transformation from cubic α phase to orthorhombic δ phase, thereby reducing the stability.

Iv. Conclusion
In summary, this study uses methylammonium halide salt (MAX, X = Cl, Br, I) as a modi er/passivator for the interlayer of PeQDs and triple-cation perovskites.CsPbI 3 PeQDs and Cs 0.05 FA 0.81 MA 0.14 PbBr 0.14 I 2.86 triple-cation perovskites and solar cell devices were prepared by hot injection and two-step methods.The effect of methylammonium halide salt on the morphology, optical properties of perovskites and device performance of PeQDs/MAX/triple-cation PeSCs were investigated.The results show that MAX salt can modify/passivate the interaction between PeQDs/CsFAMA perovskite to increase the perovskite grain size and effectively reduce the defects of perovskite lms.Compared with the control sample of pure CsFAMA, the best performance of PeSCs was achieved by using MAI salt, the Jsc of PeQDs/MAI/CsFAMA triple-cation PeSCs prepared under this condition increased from 23.0 mA cm − 2 to 24.6 mA cm − 2 , Voc from 0.938 V to 1.04, FF from 77.1-79.9%,and PCE from 16.6-20.4%.

Declarations
Figures

Figure 4 X
Figure 4

Figure 7 J
Figure 7