Thermal Properties of n = 2 and n = 3 Hybrid-Organic Perovskite Films

Abstract

Hybrid-organic materials have garnered significant attention in photovoltaic applications due to their tunable optoelectronic properties. This study explores the thermal properties of n = 2 and n = 3 hybrid-organic perovskite films, which are crucial for device fabrication. The melting behavior of these materials was investigated to understand their phase transitions and potential applications. The results reveal incongruously high melting temperatures for n = 2 and n = 3 systems compared to n = 1 layered perovskites. A melt-processing approach was employed to produce phase-pure films, overcoming challenges associated with incongruous melting.

Introduction

Hybrid-organic perovskite materials have emerged as promising candidates for photovoltaic applications due to their tunable optoelectronic properties. In particular, films with energy levels greater than n = 1 have potential utility in various devices. Thin film fabrication is commonly achieved through solution-based methods such as spin-coating, but this approach becomes problematic when working with higher energy levels (n > 2) due to the mixture of perovskite layers with different n values. An effective alternative to overcome these challenges is melt-processing, which offers flexibility in production methods.

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This study focuses on investigating the thermal properties of n = 2 and n = 3 hybrid-organic perovskite systems and explores their incongruous melting behavior at higher temperatures compared to n = 1 layered perovskites. The structural determination of these compounds remains a challenge, and understanding their thermal properties is crucial for device fabrication.

Experimental Procedure

The experimental procedure involved the synthesis of n = 1, n = 2, and n = 3 hybrid-organic perovskite films. The following reactants were used: β-Methylphenethylamine, methylammonium iodide (MAI), lead iodide, and hydriodic acid (HI) solution. For each energy level (n = 1, n = 2, and n = 3), the reactants were combined in solution and subjected to the following steps:

1. Heating at 100 degrees Celsius.
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2. Cooling at a rate of 2 degrees Celsius per hour to room temperature.

The resulting crystals were collected and washed, exhibiting distinct colors corresponding to different energy levels. The experimental techniques employed for characterizing the films included:

• Thermogravimetric analysis (TGA)
• Differential scanning calorimetry (DSC)
• X-ray diffraction measurements (XDM)
• Optical absorption measurements
• Photoluminescence measurements
• Scanning electron microscopy (SEM) for morphological analysis

A differential equation was employed to model the rate of recombination of excitons and exciton-exciton annihilation during the experiment.

Results and Discussion

Thermal Properties

The study found that both n = 2 and n = 3 hybrid-organic perovskite systems exhibited incongruously high melting temperatures when compared to n = 1 layered perovskites. X-ray diffraction (XRD) patterns did not show any discernible peaks above 250°C, suggesting that the films melted at or around this temperature.

Fabrication of Phase-Pure Films

To address the challenges posed by incongruous melting, various deposition techniques such as drop-casting, doctor-blading, and spin-coating were employed. Subsequent heating of the deposited films was performed to achieve phase-pure films. The addition of MAI and β-Methylphenethylamine (βMe-PEAI) was used to counteract the loss of organic material during the annealing process. Lower n-phase impurities were typically observed in the film when the amount of MAI was decreased or the amount of βMe-PEAI was increased. Conversely, an increase in both MAI and βMe-PEAI resulted in higher n-phase impurities in the film.

Phase-Purity Confirmation

Both n = 2 and n = 3 films underwent heating and post-annealing processes under Kapton or glass covers to facilitate phase separation and obtain phase-pure films. The recorded XRD patterns exhibited thin peaks, indicating good crystallinity. The position of the photoluminescence peak confirmed the phase purity of the films, with resulting colors matching those of their single-crystal counterparts. This supports the assertion that melt-processing, followed by post-annealing, is essential for creating phase-pure thin films.

The authors also observed that n = 2 and n = 3 systems undergo phase separation into lower n phases during the annealing process, suggesting the feasibility of using melt-processing supplemented with post-annealing to produce phase-pure films in these perovskite systems.

Conclusion

This study investigated the thermal properties of n = 2 and n = 3 hybrid-organic perovskite films and demonstrated their incongruously high melting temperatures compared to n = 1 layered perovskites. The utilization of melt-processing techniques, followed by post-annealing, allowed for the production of phase-pure films, addressing challenges associated with incongruous melting. The findings suggest the potential of these materials in various device applications, paving the way for further research in the field of hybrid-organic perovskite photovoltaics.