Shikoh Ali Sehpar
Abstract: Hybrid organic-inorganic perovskite solar cells are known to exhibit mobile ions that contribute to conductivity and hysteretic behavior of electrical and photoelectrical properties. Such (ionic) contributions can be separated from effects originating due to the presence of ordinary deep electron and hole traps in the bandgap of perovskites by combined measurements of deep level transient spectroscopy (DLTS) in both forward and reverse directions. In the case of dominant ionic contribution, mirror spectral features in DLTS and RDLTS are obtained, subsequently leading to similar activation energies. In the current work, the above-stated method is utilized to characterize solar cells based either on conventional three-dimensional (3D) perovskites or on 2D/3D perovskite heterojunction that shows superior photoelectric properties and stability. Upon characterization, the contribution of mobile ions was noted to be a leading feature for both solar cell types. This is most likely due to the low impact of electron and hole traps in good quality perovskite films. However, the signal generated due to the movement of ions was found to be significantly lower in the case of 2D/3D heterojunction perovskites. This was found to correlate with a lower amplitude of slow tails in open-circuit voltage Voc decay curves in 2D/3D solar cells, as compared to 3D solar cells. Quantitatively this assessment was done using a new method of photoinduced voltage transient spectroscopy (PIVITS), which processes the temperature dependencies of the Voc transients in the fashion similar to DLTS.