A range of electrical characterization techniques previously used for DSSC have been transferred to the study of planar perovskite devices. These include impedance spectroscopy (EIS), intensity modulated photovoltage spectroscopy (IMVS) and open-circuit voltage decay measurements (OCVD). An investigation into the observed response from these measurements has been carried out in order to gain a deeper understanding of device operation. Multiple processes with time constants on the microsecond, millisecond and second timescale were observed. The complimentary frequency and time domain techniques have been employed, showing excellent agreement between the two types of measurement. The high frequency (microsecond) process was found to be purely electronic in nature, which was linked to recombination. The geometric capacitance was shown to dominate this response, with accumulation of charge in the planar perovskite layer not observed. The lower frequency (millisecond and second timescale) processes were found to be linked to the coupling between recombination and the movement of ions. The low frequency EIS and IMVS measurements revealed that the recombination resistance was frequency dependent. The rate of change of the recombination resistance was found to be linked to the diffusion of ionic species. Activation energies for these processes were obtained (EA=0.55-0.66 eV) and shown to be in good agreement to computationally calculated values from literature for iodide vacancy migration. The same slow processes were also studied in the time domain using open-circuit photovoltage rise and decay measurements from well-defined equilibrium conditions. Comparable activation energies were also found using these techniques. The vacancy defect concentration was calculated to be 3x1019 cm-3, which is high enough for ionic double layers at the contacts to completely screen the built-in voltage across the perovskite at equilibrium in the dark. The slow dynamic processes observed under illumination or applied bias are therefore due to the rearrangement of ions in response to a changing electric field. As this rearrangement occurs, the rate of recombination is altered.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:715261 |
| Date | January 2017 |
| Creators | Pockett, Adam |
| Contributors | Cameron, Petra ; Walker, Alison |
| Publisher | University of Bath |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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