Bulk Cs 3 Bi 2 I 9 exhibits zero-dimensional (0-D) perovskite crystal structure at the molecular level, providing scopes for novel optical properties compared to three-dimensional perovskite structures. Here, 0-D refers to the crystal structure irrespective of the size of the crystal. We have prepared colloidal Cs 3 Bi 2 I 9 nanocrystals and elucidated the unique optical properties arising from their 0-D crystal structure. Absorption spectrum at 10 K confirms that the electronic band gap of Cs 3 Bi 2 I 9 nanocrystals is at 2.86 eV, along with a sharp excitonic peak at 2.56 eV, resulting in a very high excitonic binding energy, E b X = 300 meV. Interestingly, we observe two peaks in the photoluminescence spectra at room temperature on both sides of the excitonic absorption energy. Because E b X (300 meV) ≫ effective phonon energy (36 meV), the phonon-mediated relaxation of carriers from conduction band minimum to the excitonic state is suppressed to an extent. Consequently, two photoluminescence peaks related to both the bulk band edge and the excitonic transitions are observed. Furthermore, Rb 3 Bi 2 I 9 nanocrystals have also been synthesized, but they exhibit two-dimensional layered structure, unlike the 0-D structure of Cs 3 Bi 2 I 9 .

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Voltage modulated electroluminescence spectra and low frequency (≤ 100 kHz) impedance characteristics of electroluminescent diodes are studied. Voltage modulated light emission tracks the onset of observed negative capacitance at a forward bias level for each modulation frequency. Active participation of sub-bandgap defect states in minority carrier recombination dynamics is sought to explain the results. Negative capacitance is understood as a necessary dielectric response to compensate any irreversible transient changes in the minority carrier reservoir due to radiative recombinations mediated by slowly responding sub-bandgap defects. Experimentally measured variations of the in-phase component of modulated electroluminescence spectra with forward bias levels and modulation frequencies support the dynamic influence of these states in the radiative recombination process.Predominant negative sign of the in-phase component of voltage modulated electroluminescence signal further confirms the bi-molecular nature of light emission. We also discuss how these states can actually affect the net density of minority carriers available for radiative recombination. Results indicate that these sub-bandgap states can suppress external quantum efficiency of such devices under high frequency operation commonly used in optical communication.2

Articles you may be interested inPhotoreflectance and contactless electroreflectance measurements of semiconductor structures by using bright and dark configurations Rev. Sci. Instrum. 80, 096103 (2009); 10.1063/1.3213613 Temperature dependent surface photovoltage spectroscopy characterization of highly strained InGaAs/GaAs double quantum well structures grown by metal organic vapor phase epitaxy Determination of band offsets in semiconductor quantum well structures using surface photovoltage J. Appl. Phys. 94, 3955 (2003); 10.1063/1.1603346 Differential surface photovoltage spectroscopy characterization of a 1.3 μm InGaAlAs/InP vertical-cavity surfaceemitting laser structureMeasurements of electroreflectance and surface photovoltage spectroscopy of semiconductor structures are described using a transparent indium-tin-oxide-coated glass electrode in soft contact mode on the semiconductor surface. This improvisation ͑simplification͒ reduces the magnitude of the ac modulation voltage necessary for the electroreflectance measurement to less than a volt from about a kV ͑ϳ10 3 V͒ as required in the conventional contactless setup. This soft contact mode also enhances the sensitivity of the surface photovoltage signal by three orders of magnitude. We also formulate an analytical criterion to extract the transition energies of a quantum well from the surface photovoltage spectrum.

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