Fast and Facile Synthesis of Passivated Perovskite Single Crystals with Zn-Porphyrin Derivatives

Organic-inorganic trihalide, ABX3 perovskite are excellent candidates for advanced optoelectronic applications beyond photovoltaics. Perovskite single crystals offer several advantageous special features, such as a long diffusion length, low trapped carrier density, and outstanding environmental stability to use in optoelectronics. In this work, Zn has been incorporated into MAPbBr3 single crystals as Zn- Porphyrin to passivate the bulk crystals and enhance their optical and electrical properties. The inverse temperature crystallization phenomenon was used to crystallize MAPbBr3 rapidly in hot solutions. The structural and optical characterization confirms that the passivation enhanced the optoelectronic properties of the single crystals.


Introduction
Recently, there has been significant interest in the use of organic and inorganic lead halide perovskites (referred to as MAPbX3) in various optoelectronic applications, including solar cells, light-emitting diodes, photodetectors, and other devices [1][2][3], light-emitting diodes [4], photodetectors [5], and other devices.These materials possess advantageous characteristics such as a precisely controllable band gap, a high light-absorption coefficient, an extensive carrier transport distance, and a minimal number of defects [6][7][8].Additionally, they can be deposited using low-cost and uncomplicated techniques.To enhance the performance of these devices, numerous endeavors have been undertaken to produce highquality perovskite materials, such as polycrystalline films or single crystals [9,10].
Single-crystal (SC) perovskite materials possess numerous desirable properties, including a long diffusion length, a low trapped carrier density, and exceptional environmental stability [11].Their minimal defect densities and absence of grain boundaries make them suitable candidates for optoelectronic devices.Among the various members of the MAPbX3 family, MAPbBr3 is well-known for its stability compared to MAPbI3 and its wider range of visible light absorption compared to MAPbCl3.Furthermore, MAPbBr3 exhibits cubic crystal symmetry, which facilitates the formation of high-quality single crystals [12].Bulk MAPbBr3 single crystals are grown using various techniques such as laser-induced localized growth, antisolvent vapor-assisted crystallization, and inverse temperature crystallization (ITC) [13].However, these reported methods for perovskite single crystallization have certain drawbacks, including slow growth rates for large-scale applications or unsatisfactory quality for demanding optoelectronic applications [14].The development of large-scale perovskite single crystals with rapid growth and excellent quality would enable broader applications.The ITC method, which relies on the inverse solubility of perovskite halides in specific solvents, has emerged as a popular approach for achieving rapid growth of large-scale, high-quality perovskite single crystals.This process takes advantage of the temperature-dependent solubility of perovskite molecules in the solvent.Unlike conventional dissolution processes, where perovskite molecules dissolve freely, most of the perovskite molecules form complexes with the solvent, reducing the number of unbound molecules.At lower temperatures, the complexes remain stable, and the solution is not saturated with unbound molecules.However, as the temperature increases, the binding energy of the complexes decreases.Eventually, the solution becomes supersaturated, causing the unbound perovskite molecules to rise and initiate crystallization.
The presence of defects and trap states in perovskite materials can hinder charge carrier mobility and lead to charge carrier recombination, thereby reducing the efficiency of solar cells.Researchers have sought to address this issue by employing chemical passivation techniques to fill in surface dangling bonds and reduce the density of defects and trap states [15][16][17].Recently, porphyrin derivatives have emerged as promising additives in perovskite optoelectronics due to their high light absorption, favorable charge transport properties, and excellent thermal stability [18,19].To achieve highperformance optoelectronic devices, the advantages of multifunctional porphyrins in defect passivation need to be fully exploited.
Bulk passivation, which involves passivating the entire crystal during the growth process, offers advantages over surface passivation, particularly in terms of controlling growth defects and improving trap density.In this study, zinc (Zn) was incorporated into MAPbBr3 single crystals as Zn-Porphyrin (Zn-PP) to passivate the bulk crystal and enhance its optical and electrical properties.Large-scale and high-quality single crystals of MAPbBr3 were successfully produced using the inverse temperature crystallization (ITC) method, taking advantage of the high solubility of hybrid perovskites at low temperatures.The structural, optical, and electrical properties of the single crystals were extensively investigated as crucial parameters influencing device performance.Moreover, through analysis of charge transport characteristics, the synthesized perovskite single crystal demonstrated potential for application in photodetectors.The low-temperature crystallization process offers a scalable approach for generating numerous high-quality perovskite single crystals, thus facilitating their use in various perovskite-based applications.

Experimental Method
The inverse temperature crystallization method was employed to grow single crystals of methylammonium lead bromide (MAPbBr3) [20].To prepare the growth solution, methylammonium bromide (MABr) and lead bromide (PbBr2) were dissolved in N,N-dimethylformamide (DMF) with a 1:1 molar ratio.The solution was continuously stirred at room temperature until complete dissolution.Subsequently, the solution was filtered using a 0.2 μm PTFE filter to remove any impurities.After filtration, the transparent solution was divided into 2 mL portions and placed in vials, which were then immersed in an oil bath.The oil bath was maintained at a temperature of 80°C throughout the process.For the preparation of MAPbBr3 single crystals passivated with Zn-PP, 0.1 wt% of Zn-PP was added to the solution described above.The crystals used for measurements were grown by transferring seed crystals into freshly prepared solutions.The crystallization procedure was repeated every 3 hours to facilitate the growth of single crystals with a size of approximately 5 mm.
All the procedures were conducted under ambient conditions with a humidity level of 55-60%.The optical and structural properties of the single crystals were analyzed using various techniques.Photoluminescence spectroscopy (PL) was employed to investigate the emission properties of the crystals upon excitation.UV-Vis-NIR transmittance spectroscopy was used to study the absorption properties of the crystals across a wide range of wavelengths.X-ray diffraction (XRD) measurements were conducted to determine the crystal structure, lattice parameters, and overall crystal quality.These techniques provided valuable insights into the optical and structural characteristics of the MAPbBr3 single crystals.
To fabricate planar photodetectors, interdigital Au electrodes were deposited on the top surface of the MAPbBr3 single crystals.The deposition was carried out using the vacuum evaporation method, resulting in Au electrodes with a thickness of approximately 100 nm.Each electrode was composed of a group of fine Au wires, which allowed for further investigation of the device performance.The effective illuminated area of each photodetector was approximately 0.84×10 -3 cm 2 .This area represents the region on the surface of the photodetector that receives light and contributes to the device's photo response.

Results and Discussion
At elevated temperatures (80°C) and concentrated precursor solutions (1M) containing equal amounts of Methylammonium bromide (MABr) and lead bromide (PbBr2), we observed the rapid formation of small MAPbBr3 perovskite precipitates.However, by carefully controlling the temperature and concentration of the precursor in N,N-dimethylformamide (DMF) at 80°C and 1M, respectively, we were able to produce only a few crystals.To synthesize MAPbBr3 single crystals passivated with Zn porphyrin (Zn-PP), we combined the perovskite solution with Zn-PP using the same method.The concentration of Zn-PP added to the precursor solutions was expressed as a weight percentage relative to the PbBr2 content.
As a result of the crystallization reaction at 80°C, we successfully obtained 5mm-size MAPbBr3 single crystal cubes, as shown in Figure 1(a).Each individual MAPbBr3 crystal was found to grow at a rate of approximately 40 mm 3 /h, which is significantly faster than the growth rates reported in previous methods [21].Notably, we observed that the crystallization process is reversible for both MAPbBr3 single crystals and Zn-PP passivated MAPbBr3 single crystals.When the crystals were cooled back to room temperature, they dissolved, indicating the reversible nature of the crystallization process.The UV-Vis-NIR transmission spectrum of the unprocessed and bulk-passivated Zn-PP MAPbBr3 SCs is shown in Fig. 1(b).After Zn-PP passivation, a substantial increase in transmittance was seen.This proves that Zn-PP has an impact on the optical characteristics of MAPbBr3 crystals.Using the Tauc-plot method, the band gap of the crystal was calculated and was found to be 2.216 eV for the untreated samples and 2.203 eV for the Zn-PP treated samples, respectively.The slight reduction in bandgap in Zn-passivated perovskite single crystals suggests improved crystallinity [15] with Zn-PP, which is further supported by the XRD high-intensity peaks for the passivated perovskite single crystals relative to the nontreated perovskite single crystals as seen in Fig. 2(a).Using photoluminescence spectroscopy, the optical characteristics of the single crystals were further assessed.The PL spectrum of the SCs with and without Zn-PP passivation is depicted in Fig. 2(b).It is noticed that the PL emission peak position of Zn-PP passivated MAPbBr3 SCs exhibits a 7 nm red-shift under similar conditions.This observation may be explained by the larger ion radii of Zn-PP than Pb, which causes the change in the lattice.An important point to remember is that the addition of Zn-PP to MAPbBr3 single crystals passivates the bulk perovskite and decreases defects, leading to a notably higher PL peak intensity.The improvement in crystallinity caused by the Zn-PP derivative is due to this red shifting, which is supported by the XRD measurement in Fig. 2(a).

Conclusion
In this work, we successfully synthesized quickly crystalizing, a few millimeters in size pristine MAPbBr3 single crystals and Zn Porphyrin passivated single crystals.In this inverse temperature crystallization method, the precursor solution concentration and crystallization temperature play a vital role in determining the rate and size of a single crystal in a hot solution.A few mm size MAPbBr3 single crystals cubes were obtained after crystallization reaction at 80°C.A significant increase in the PL peak intensity after the incorporation of Zn-PP in the MAPbBr3 single crystals passivates the bulk perovskite and reduces the defects.It is confirmed by the optical and structural characterization that the optical properties of the MAPbBr3 SCs can be modified by introducing Zn-PP, without destroying the perovskite single crystal structure.By introducing bulk passivation on the MAPbBr3 single crystal, the photo response performance of the photodetector has improved significantly.This study demonstrates the passivation of MAPbBr3 by Zn-PP SCs using solution-processed growth is a viable method for improving the optoelectronic characteristics of photo-detector performance in the future.

Fig. 2 (
Fig. 2(a) XRD measurements of the pristine and Zn-PP passivated single crystals.(b) PL spectrum of pristine and Zn-PP passivated SCs

Fig. 3 (
Fig. 3(a) Photocurrents of devices at different voltages.Photocurrent response of MAPbBr3 Photodetector measured under 532 nm switching light of (b) pristine and (c) passivated Photodetector device.

Fig. 3 (
Fig. 3(a) shows the dark current of the MAPbBr3 and Zn-PP passivated MAPbBr3 photodetector.It was observed that the dark current of the Zn-PP passivated sample was significantly lower than the unpassivated MAPbBr3 sample.The decrease in dark current is due to the effective defect passivation of MAPbBr3 single crystal.Even at lower light intensities, this detector offers a high on-off ratio and an excellent linear response over a wide range of light intensities.This suggests that the detector has improved in terms of photo response performance.The rate of response of a photosensor is strongly influenced by charge collection and transport.The present photosensor's transient photocurrent responsiveness was measured under 532 nm chopped laser light.The rising time (trise) and fall time (tdecay) of unpassivated MAPbBr3 SC photodetector was found to be 2.1 μs and 46.60 μs respectively as shown in Fig 3(b).The trise and tdecay of Zn-PP passivated MAPbBr3 SC photodetector was found to be 0.55 μs and 43.71 μs respectively as depicted in Fig 3(c).It is obvious that the Zn-PP passivation improved the Photo response characteristics of MAPbBr3.The devices show a certain amount of