Characterization and C-DLTS analysis of antimony selenide solar cells

In this work, we report on the performance and characteristics of ASe solar cells. To describe the presence of defects, we carried out a capacitance deep-level transient spectroscopy (C-DLTS) of copper-doped antimony selenide solar cells with the following structure: AZO/ZnO/CdS/Cu:Sb2Se3/FTO/Glass. The experimental data highlight the non-ideal behavior of the devices, and indicate that: (i) the rate window analysis, conventionally used in DLTS, does not provide reliable information; charge trapping mechanisms need to be identified through capacitance-transient measurements; (ii) the electron injection in the active region highly depends on the filling voltage (Vf); (iii) carriers de-trapping kinetics strongly depend on the duration of the filling pulse (tf). First, we demonstrate that the trap filling process is stronger at higher temperature, which leads to higher amplitude of the capacitance transient. ∆C is then mainly associated to a higher density of captured carriers, rather than a change in the emission time constant τ. Second, we carried out a series of measurements in which both tf and Vf were varied at a fixed temperature of 140 K. Results showed that larger filling voltages Vf correspond to higher capacitance variation ∆Cs, and the increase is visible also when tf is increased. Both findings can be explained by the higher density of majority carriers in the SCR as the two parameters increase. On the other hand, the effect of the minority carrier injection starts to be visible only for positive values of Vf, as expected. This is confirmed by the presence of minority carriers transients even at low tf for high Vf. Finally, we showed that the time constant of the carrier de-trapping process increases with increasing duration of the filling pulse tf; this is explained considering that with a longer filling time more carriers can be trapped in the SCR, and they can reach regions farther away; transport becomes then relevant in the emission process. In summary, trap-characterization in ASe cells requires the use of unconventional methodologies, to reach accurate data interpretation. The results emphasize the significance of capacitance transient measurements in accurately identifying charge trapping mechanisms, necessary for the development and optimization of the devices.