Researchers using the unique “mail-in program” to submit samples at the XSD 11-BM-B beamline at the APS are exploring the properties of colloidal nanoparticles of pervoskites for technological applications. Organometal halide perovskites have attracted considerable attention as a solution for low-cost opto-electronic devices. The findings have been published in The Journal of Physical Chemistry Letters.
Characteristics of nano-scale materials are often different from the corresponding bulk properties providing new, sometimes unexpected, opportunities for applications. Here we investigate the properties of 8 nm colloidal nanoparticles of MAPbBr3 perovskites and contrast them to the ones of large micro-crystallites representing a bulk. X-ray spectroscopies provide an exciton binding energy of 0.32±0.1 eV in the nanoparticles. This is 5 times higher than the value of bulk crystals (0.084±0.010 eV), and readily explains the high fluorescence quantum yield in nanoparticles.
In the bulk, at high excitation concentrations, the fluorescence intensity has quadratic behavior following the Saha-Langmuir model due to the non-geminate recombination of charges forming the emissive exciton states. In the nanoparticles, a linear dependence is observed since the excitation concentration per particle is significantly less than one. Even the bulk shows linear emission intensity dependence at lower excitation concentrations. In this case the average excitation spacing becomes larger than the carrier diffusion length suppressing the non-geminate recombination. From these considerations we obtain the charge carrier diffusion length in MAPbBr3 of 100 nm.
Kaibo Zheng, Qiushi Zhu, Mohamed Abdellah, Maria E Messing, Wei Zhang, Alexander V. Generalov, Yuran Niu, Lynn Ribaud, Sophie E. Canton and Tonu Pullerits, “Exciton Binding Energy and the Nature of Emissive States in Organometal Halide Perovskites,” The Journal of Physical Chemistry Letters, Just Accepted Manuscript. DOI: 10.1021/acs.jpclett.5b01252. Published Online July 14, 2015.