Low-Frequency DLTS of Perovskite Solar Cells with the MFIA

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 with MF-IA option are perfect instruments to sit at the heart of a DLTS system in order to acquire these critical capacitance transients at variable test signal frequency and variable test signal amplitude.

This blog post introduces recent work carried out using the MFLI with MF-IA by Dr Sebastian Reichert in the group of Prof Carsten Deibel at the Technical University of Chemnitz. They show the advantages of reducing the test signal frequency to 80 kHz and test signal amplitude to 20 mV. It also highlights the importance of being able to acquire both fast transients (10 µs) and also the longer transients (30 s).

Dr Reichert continues the introduction…

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 allows 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 5K steps for all perovskite solar cells with a stoichiometric ratio of 1:3.00. The transients were normalized with the equilibrium capacitance C0 and measured over 30 s.

By combining the results of IS and DLTS measurements, we identify three ionic defects, which we attribute to VMA, Ii, 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 find that the ionic defects we observed fulfil the Meyer–Neldel rule, which allows to categorize all observed ionic defects. Our results offer significant insights into the defect physics of perovskite materials and progress the current understanding of the underlying processes that govern the properties of this phenomenal class of materials.

Figure 2: Literature comparison; Migration rates reported in literature were 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.

Read the full paper…

To read the full story, you can find the complete free-to-access paper on Nature Communications here:

Probing the ionic defect landscape in halide perovskite solar cells
Sebastian Reichert, Qingzhi An, Young-Won Woo, Aron Walsh, Yana Vaynzof & Carsten Deibel
Nature Communications volume 11, Article number: 6098 (2020)

Thank you

Congratulations to Dr Reichert, Prof Deibel and the whole team for this groundbreaking work on low-frequency DLTS, and many thanks for sharing this study.