Development of Inorganic Perovskite Nanocrystals for Optoelectronic Applications

Abstract

Metal Halide Perovskites (APbX3) have garnered appreciable attention from researchers because of their fascinating optoelectronic properties. These properties include high absorption coefficient, defect tolerance, long carrier lifetime, long charge carrier diffusion length and tuneable emission in the visible region (400 -700 nm). As a result of these properties, they are being extensively used in optoelectronic applications such as solar cells, LEDs, photodetectors, lasers and scintillators. Over the years the power conversion efficiency of solar cells has increased from 3.8 % to 26.0 %. The External Quantum Efficiency (EQE) of perovskite-based light emitting diodes is over 28 %. Properties such as power conversion efficiency and EQE are expected to increase manifolds when the size and dimensionality reduces to nm range due to increased exciton binding energy owing to quantum confinement effects. Unlike the conventional semiconductor quantum dots which require narrow size distribution of the core and electronic passivation of an outer shell, perovskites are efficient emitters even without core-shell architecture. The emission from the perovskite nanocrystals (PNCs) can be tuned over the visible range by adjusting the halide composition (Cl, Br, I) and particle size (quantum size effects). Contrary to chalcogenide NCs, the surface dangling bonds do not result in mid-gap trap states, and the perovskites emit efficiently even without surface passivation. This unique property is called defect tolerance where the defects are benign to the optical properties of the Metal Halide Perovskites. However, perovskites are susceptible to external conditions such as moisture, heat, oxygen, light and UV which leads to their easy degradation and hence makes their incorporation into device a challenging task. Inorganic (Cs) lead halide perovskites (IHPs) show improved thermal stability from their organic (MA+, FA+) counterparts owing to their high thermal decomposition temperatures and show better optical characteristics. Among the IHPs, CsPbBr3 shows exceptional optoelectronic properties among CsPbCl3 and CsPbI3 which includes narrow linewidth, high photoluminescence quantum yield, short radiative lifetime, and enhanced stability. In addition to 3D CsPbBr3, the photoluminescence of its 0D counterpart Cs4PbBr6 has also been the subject of intense debate. Whether the emission originates from the presence of 3D CsPbBr3 impurity, or the presence of Br vacancies or self-trapped excitons has been the centre of intensive research. Also, the co-existence of both 3D and 0D phase brings vagueness in solving the emission riddle. Amidst the plethora of research going on IHPs, the presence of lead (Pb) cannot be ignored. Lead has been the central character behind the fascinating properties of perovskites. But Lead has long term toxic effects on both ecology and humanity. Hence, research has taken a new direction in the synthesis of lead-free perovskites where lead is substituted with a less toxic element while keeping the optical properties intact. This thesis is divided into two parts. In the first part, mixed phase PNCs (CsBr, Cs4PbBr6 and CsPbBr3) are synthesized and the emission is tuned from blue to green using colloidal route. Blue- emitting Cs4PbBr6 is synthesized, and its emission origin is discussed. Further, non-linear optical studies are performed on CsPbBr3 Nanocrystals (NCs). In the second part, In- based perovskite is synthesized emitting in 430- 460 nm region. Both lead based and lead-free PNCs show superior stability which is a long-standing hurdle in perovskite research. Hence, the PNCs can find applications in nano LEDs, displays and non-linear optical devices. The research work discussed in this thesis are as follows:

• The precursor ratio Cs: Pb influences the phase and emission forming mixed phase PNCs (CsBr, Cs4PbBr6 and CsPbBr3) and the emission can be tuned from blue to green. The mixed phase PNCs show broad emission from multiple recombination centers in the 400- 700 nm region. In addition, the synthesized PNCs are highly stable and maintain their phase and emission even after two years of synthesis when stored in low temperature.

• Optimization of injection temperature leads to pure 0D Cs4PbBr6 NCs. With the change in injection temperature impurity CsPbBr3 grows which changes the emission color from blue to green. The origin of emission from 0D phase is discussed. The CIE coordinates of 0D Cs4PbBr6 is (0.14,0.09) which is in line with the NTSC standard for blue emitter. In-depth stability studies are conducted in ambient conditions, under constant UV illumination and under refrigeration. The PNCs are stable even after two years of synthesis when kept under refrigeration.

• The role of ligands on the growth of phase and emission of the mixed phase perovskite PNCs is studied.

• CsPbBr3 PNCs with narrow fwhm and high PLQY of 71.42% are synthesized and nonlinear optical studies are conducted. The PNCs show both SHG and THG signals along with superior stability.

• This work concludes with the facile synthesis of Lead-Free Perovskites (LFP). A group 13 element (Indium, In) is chosen as the lead substitute. Cs3InBr6 LFP NCs shows emission in the blue region (430-460nm) with a PLQY of 6.08%. Stability studies are performed on the LFP NCs in the temperature region -10 °C to 50 °C, ambient conditions, and UV illumination. In addition to In, Silver (Ag) was also used to synthesize LFP NCs. Different molar ratios of Ag and In was used to study the material and optical properties. However, the emission originated from Cs3InBr6 LFP NCs and different molar ratio of Ag and In lead to the formation of secondary phases.