1, Iowa State University, Ames, Iowa, United States
3, National Chemical Laboratory, Pune, , India
The methyl ammonium halide family of perovskite materials has revolutionized solar energy conversion, generating power conversion efficiencies exceeding 22%. In addition to promise for solar energy conversion, there is novel materials physics of these perovskite materials. One example are the existence and interplay of excitons and charge carriers are which are key fundamental properties for emerging photovoltaic applications. Despite intense studies it is still challenging to yield information on intrinsic transport parameters and their fs dynamics, such as carrier mobility, density and the initial dynamic pathways of exciton formation and decay. There have been very limited research efforts to provide simultaneously local microstructure measurement and fs carrier dynamics in the same perovskite samples. Here we perform a comprehensive, spatial–temporal spectroscopy characterization of the methyl ammonium perovskite samples using laser-scanning confocal microscopy and ultrafast terahertz spectroscopy- which demonstrate a powerful and versatile approach to fully characterize response functions of excitons and charge carriers, and intrinsic transport properties. Our results show photo-luminescence quenching and lifetime variations due to the impact of local heterogeneity. The samples also show multiple sharp quantum transitions from excitonic Rydberg states at 10.1 meV and 12.1 meV assigned to 1s-2p and 1s-3p internal transitions, characterizing weakly bound excitons with binding energy ~13.5 meV. Transient carrier and exciton populations are precisely determined using ultrafast terahertz conductivity, which is superior to conventional linear optical measurements.
To obtain fundamental insights into excitonic states, we computed the electronic band structure, carrier effective masses, and optical properties using ab-initio density functional theory. Simulations with spin-orbit coupling were necessary to provide reasonable effective masses and exciton binding energies. Simulations give a direct band gap of 1.58 eV, and a dielectric constant ~18, indicating high dielectric screening of internal quantum states leaving excitons weakly bound. As found in other calculations, heavy electrons and light holes combine to provide effective masses.
The excitonic/carrier dynamics and complementary spatial–temporal spectroscopy methods demonstrated shine new light on fundamental perovskite materials physics that have clear implications towards their applications in photovoltaics and optoelectronics.
Supported by U.S. DOE Office of Science, Basic Energy Sciences, Materials Science and Engineering Division.