| Abstract [eng] |
The purpose of thesis work is to investigate charge carrier dynamics in polycrystalline MAPbI3 perovskites using nonstationary photocurrent methods. Perovskites have attracted great attention in scientific community due to long diffusion lengths, high absorption coefficient, tunable band gap and their synthesis is relative cheap. These properties make these materials, especially metalorganic perovskites, competitive to existing III-V semiconductors, such as silicon or gallium nitride, in optoelectronic device area like solar cells or LEDs. Due to polycrystalline structure, perovskites tend to have high trap densities on surface or in grain boundaries which highly affect carrier dynamics in devices. The latter mentioned location of defects have dual properties which can trap carriers or effectively separate electron and hole pairs. For this research, a solution of methylamonium iodide (MAI) and lead iodide (PbI2) have been deposited on platinum electrodes by spin coating method. Two samples with different ratios of reagents have been produced. Photocurrent kinetics of the samples have been measured by exciting with Nd:YAG nanosecond pulsed laser and inducing electrical field by function generator where signal has been registered with oscilloscope. Laser wavelength was λ = 532 nm, 10 Hz frequency and duration of periodical electrical pulses was 25 ms. Photocurrent dependence on various voltage and laser intensity values have been measured where intensity has been varied from 1 nJ/cm2 to 4300 nJ/cm2. Electrical field strength has been varied from 0.05V to 5V (increasing every voltage value by double). Measured photocurrent kinetics have been numerically modeled to derive parameters such as photocurrent density j, charge density n, carrier mobility µ and trapping coefficient k. Photocurrent dependency on laser intensity of different MAPI samples with constant voltage value have indicated photocurrent dependency on material composition. Similar dependency between photocurrent and perovskite composition have been derived from photocurrent measurements with different voltage while laser intensity value was constant. Time-delayed collection field method measured photocurrent with delayed application of voltage which showed reduced charge mobility which affected charge extraction. Reduction of mobility values was caused by grain boundaries which created potential barriers which restricts carrier movement between grains when electrical field is applied. Numerical modelling have showed reduced mobility values which is 3 times less for sample with bigger MAI quantity in perovskite composition. Model of charge carrier dynamics in polycrystalline MAPI perovskites has been proposed which explained underlying mechanisms of electron and holes movement under electrical field in the presence of potential barriers. |
