Sandesha Nayak

Senior research associate, Syngene International Ltd, Bengaluru, India

sandeshanayak10023@gmail.com


Introduction

A junction diode allows current to flow when electrons and holes move across the boundary between n-type and p-type semiconductor. An LED is a junction diode with an added feature of producing light. Two slabs of semiconducting material i.e., n-type and p-type are joined together. When the junction is forward biased, the electrons and holes move towards the junction and recombine forming an exciton. The decay of the exciton is accompanied by emission of radiation whose frequency is in the visible region. There are 2 types of LEDs based on chemical composition, organic LEDs (OLEDs) and inorganic LEDs (ILEDs). The disadvantages of the OLEDs are limited lifetime- short term battery life, manufacturing expense and water easily damaging OLEDs. To overcome these disadvantages inorganic LEDs are better option. The life spans of inorganic LEDs much longer than OLEDs.

Perovskite materials have attracted wide attention because of the unique optical, thermal, and electromagnetic properties. Sushant Ghimire et al., showed amplified emission and multicolour emissions from interfacial layers of chloride and bromide perovskites which are promising candidate for LEDs. The metal halide perovskite possess a generic formula of ABX3, were A is monovalent cation (MA+ = CH3NH3+,  CH(NH2)2+ and cesium (Cs+) ; B is a divalent metal cation such as lead (Pb2+) and tin (Sn2+ ) ions, and X is a halogen anion such as Cl, Br, I. These perovskite nanocrystals show  great application in many optoelectronic devices, such as light emitting diodes (LEDs) and photo detectors.

To develop the stability of the perovskite solar cells, inorganic perovskites into photovoltaic devices was proposed, due to their  stability, especially at high temperature. In recent years, various structures of perovskite solar cells have been developed including the mesoporous structure and the planar heterostructure. The perovskite materials in solar cells play a major role in absorption of light and photoelectric conversion. Perovskite solar cells aim to increase the efficiency and lower the cost of solar energy. The  significance of perovskite solar cells is generating photoelectric energy from light whereas LEDs emit light from electric energy.

Objectives

  • Synthesis of novel inorganic silver-based perovskite nano crystals because of their higher stability compared with the hybrid organic–inorganic perovskites and lead based peroviskte nano crystals.
  • Structural characterization  of inorganic silver based perovskites by  powder X-Ray diffraction and high resolution transmission electron microscopy (HRTEM).
  • Optical properties studies by UV-VIS absorption and photoluminescence spectroscopies.
  • Elemental Analysis by high angle-annular dark field (HAADF) HRTEM.
  • Thermal stability studies by thermo gravimetric analysis.

Experimental section

Method 1: Synthesis of CsAgCl3 and CsAgBr3

Preparation of  CsAgCl3

2AgCl2 + 2HCl + Cs2CO3 ==> 2CsAgCl3 + 2HCO3

The cesium silver chloride powdered perovskite can be  synthesized by dissolving 1 mmol  of silver chloride (AgCl2) in 10 mmol of hydrogen chloride at room temperature. After complete dissolution of the silver chloride,  1 mmol of cesium chloride was added. The mixture was stirred for 30 minutes to ensure the completion of reaction. The obtained precipitate was filtered and washed with ethanol. 

Preparation of CsAgBr3

2AgBr2 + 2HBr + Cs2CO3 ==> 2CsAgBr3 + 2HCO3

The cesium silver bromide may be prepared by dissolving 1mmol  of silver  chloride in 5cm3 of hydrogen bromide (HBr) followed by the addition of equimolar amount of cesium bromide in a round bottom flask. The reaction mixture was stirred for one hour to ensure the completion of reaction. The resulting mixture was kept at room temperature for one day to obtain precipitates which were filtered and washed with ethanol.
 
Method 2: Synthesis of CsAgX3

Synthesis of Cs-oleate as a cesium precursor:  To a solution of Cs2CO3 (1 equivalent) in octadecane was added oleic acid, stirred under vacuum for 30mins at 1200C, and then heated under N2 atmosphere to 1500C until Cs2COreact with oleic acid. Cs-oleate precipitates out of octadecane at room temperature.

Synthesis and purification of CsAgX3: To a solution of AgX2 (0.9 equivalent) in octadecane was added OLAM and oleic acid at 1290C under Nitrogen atmosphere. After degassing temperature raised to 3150C and prepared Cs-oleate solution was injected. After adding temperature raised to 3200C for 1hour. Cooled and purified by high-speed centrifugation.

Conclusion

This study on inorganic hybrid perovskite materials and devices is a step in the right direction  to find a long lasting solution to the stability needs of industry and society. The precursor materials are inexpensive and at the same time easy to synthesize. Thus, from the production point of view, devices based on these materials will ultimately lead to less expensive display technology. On the other hand, results will stimulate further research and development in other fields beside optoelectronics, materials science, materials chemistry, solid-state and physics. This will have the way for the development of new materials with fascinating physics and chemistry. It will also lead to development of other electronic devices.


 References 

1. Akkerman, Q.A., et al., “Tuning the Optical Properties of Cesium Lead Halide Perovskite Nanocrystals by Anion Exchange Reactions”. Journal of the American Chemical Society, 2015. 137(32).

2. Li, X., et al., “All Inorganic Halide Perovskites Nanosystem: Synthesis, Structural Features, Optical Properties and Optoelectronic Applications”. Small, 2017. 13(9).

3. Schmidt, L.C., et al., “Nontemplate Synthesis of CH3NH3PbBr3 Perovskite Nanoparticles”. Journal of the American Chemical Society, 2014. 136(3).

4. Huang, H., et al., “Emulsion Synthesis of Size-Tunable CH3NH3PbBr3 Quantum Dots: An Alternative Route toward Efficient Light-Emitting Diodes”. ACS Applied Materials & Interfaces, 2015. 7(51).

5. Palazon, F., et al., “Polymer-Free Films of Inorganic Halide Perovskite Nanocrystals as UV-to-White Color-Conversion Layers in LEDs”. Chemistry of Materials, 2016. 28(9): p. 2902-2906.


About the author

Mr Sandesha Nayak is Senior research associate at Syngene International Ltd, Bengaluru, India from past four years.