Deep level transient spectroscopy (DLTS) is a powerful and commonly used technique to investigate the concentration and carrier binding energy of defects in semiconductors. The technique involves measuring capacitance transients at different temperatures, typically at 1 MHz. However, DLTS can also be carried out at lower frequencies such as 80 kHz to probe slower processes. The MFIA Impedance Analyzer and the MFLI Lock-in Amplifier with the MF-IA option are perfect instruments to sit at the heart of a DLTS system in order to acquire critical capacitance transients at variable test signal frequency and variable test signal amplitude.
This blog post introduces work carried out by Dr. Sebastian Reichert, in the group of Prof. Carsten Deibel at the Chemnitz University of Technology, using the MFLI with the MF-IA option. Dr. Reichert and colleagues show the advantages of reducing the test signal frequency to 80 kHz and the test signal amplitude to 20 mV. Their work also highlights the importance of being able to acquire fast transients (10 µs) and longer transients (30 s) too. Read on for Dr. Reichert's comments on their results.
Semiconductors based on the perovskite structure are promising candidates for renewable energy conversion. Point defects in metal halide perovskites play a critical role in determining their properties and optoelectronic performance. Unfortunately, many open questions remain unanswered. We investigated the ionic defect landscape of MAPbI3 solar cells. All measurements, including deep-level transient spectroscopy (DLTS) and impedance spectroscopy (IS), were carried out with the Zurich Instruments MFLI Lock-in Amplifier with MF-IA and MF-MD options, which allow an accurate and reliable determination of the device’s capacitance. For probing the capacitance in DLTS, we used a frequency of 80 kHz instead of the common 1 MHz. We think that the organic transport layers limit the probe frequency for the photoactive perovskite layer. The usual transient length is from 10 µs to 30 s.
Figure 1: DLTS measurements for different temperatures from 200 K to 350 K in 5 iedK steps for all perovskite solar cells with a stoichiometric ratio of 1:3.00. The transients are normalized with the equilibrium capacitance C0 and measured over 30 s.
By combining the results of IS and DLTS measurements, we identified three ionic defects, which we attribute to V−MA, I−i, and MA+i. We showed that mobile ions accumulate at the interfaces to the transport layers, where they influence the electrical properties of the solar cell. The presence of ionic interfacial layers is also shown to affect the EA of the various defects, by impeding their transport due to the high electric fields they introduce. We compared the temperature-dependent ion migration rates to the literature, and were able to categorize defect parameters of different perovskite materials and device architectures. Importantly, we found that the ionic defects we observed fulfil the Meyer–Neldel rule, which makes it possible to categorize all observed ionic defects. Our results offer significant insights into the defect physics of perovskite materials and move forward our current understanding of the underlying processes that govern the properties of this phenomenal class of materials.
Figure 2: Migration rates reported in the literature are plotted in an Arrhenius diagram for comparison to our findings (species β (crosses), γ (circles), δ (stars)). All reported emission rates can be associated with two regimes, at low emission rates and high temperatures and at high emission rates at high and middle temperatures.
Here is the fully accessible paper: Reichert, S. et al. Probing the ionic defect landscape in halide perovskite solar cells. Nat. Commun. 11, 6098 (2020).
Acknowledgments: Many thanks for Dr. Reichert, Prof. Deibel and the rest of the team for sharing this study of low-frequency DLTS.