Investigating ion transport in semiconducting materials for energy conversion

Lead Research Organisation: Imperial College London
Department Name: Physics

Abstract

Semiconductors which allow the transport of mobile ions are important in devices used for energy conversion and harvesting, such as batteries and solar cells, as well as of great potential interest for devices such as memristors or memtransistors that could have low power neuromorphic computing applications.

This project will investigate the behaviour of electronic devices fabricated from semiconductors that allow ion transport, an area that is not currently well understood. The initially focus will be on hybrid perovskite semiconductors which show high quality electronic and optical properties despite being prepared from solution based precursor materials at low temperature. Solar cells made with these perovskites perform well. However they often show hysteresis in their current-voltage performance which can be explained by the migration of ionic defects within the semiconductor to screen the internal electric field. Although this behaviour is problematic for solar cells it is potentially beneficial for other classes of electronic device. Very recent work has demonstrated that two terminal hybrid perovskite devices can show behaviour similar to that of a synapse between neurons. Interest in neuromorphic computing and synaptic electronics is remerging due to its potential for performing energy efficient, fault tolerant computation in compact spaces.

The early stages of the project will involve the design, fabrication and characterisation of electronic devices, made from hybrid perovskites, which show a continuously varying memory of their previous operating history. An aim is to explore two terminal (memristor or memdiode), three terminal (memtransistor) as well as more exotic device architectures suitable for application in new parallel neuromorphic computing. Key challenges will be to develop approaches to control the relationship between material properties, contacts, device behaviour and stability. The project will also examine the characterisation and understanding of ion penetration in oxide semiconductors used for battery electrodes.
 
Description One project I have been working on involves applying a series of electrical potentials of differing size and timescales across solar cells and memristor/single carrier devices. The objective of this project is to uncover the timescale of the impact of ionic motion on electronic charge for different device structures and apply this to the analysis of current density-voltage plots, namely space-charge limited current (SCLC). The voltage pulses, if of the correct length of time, would cause a response from just electrons/ holes and not the ionic charge (as they are expected to have very different mobilities). This work also helps verify frequency dependent measurements (for example impedance spectroscopy) on solar cell devices by allowing a direct measurement of the timescale of the response of the device. So far this work has shown that SCLC analysis is not valid in most perovskite device architectures (proved through simulation and experiment) and it shows that the response time of perovskite devices depends strongly on the device architecture due to the equilibrium position of the mobile ionic charge before measurement during voltage pulses.
Another project I am working on is determining the change in ionic conductivity of different perovskite solar cell devices under different environmental humidities. This will possibly verify theoretical work done on the effect of water on the crystal lattice of perovskites by measuring the impact this has on ionic conductivity. This, in turn, could demonstrate another technique for controlling ionic conductivity from the fabrication process and also unveil why we see different levels of stability under humidity of different perovskite compositions. The early results show that water does indeed intercalate into the perovskite crystal lattice and increase the ionic conductivity but the results need to be verified with a reproducibility study. The study also shows that different perovskite compositions show different changes in conductivity with humidity.
Other projects I have been working on include: finding an equivalent circuit for describing perovskite field effect transistors (FETs) which could provide a tool for finding measurement and fabrication techniques for improving perovskite FET performance; investigating alternative contact layers with perovskites that could selectively remove mobile ions from the crystal structure under different applied biases and testing the constraints of equivalent circuit models with intensity modulated photocurrent/photovoltage spectroscopy.
The key findings for these projects are still under analysis but, if all goes well, I anticipate some publications to come from this work.
Exploitation Route The outcomes of this funding will greatly effect the understanding of how mobile ionic species can be controlled - both to the advantage of memristors and potential disadvantage of solar cells. The ionic species impact many areas of analysis for perovskite devices so this research will influence fabrication techniques and operational applications of hybrid perovskites, hopefully improving the material's viability in the market for many opto-electronic applications. The research will help future researchers at my institution and other relevant institutions globally, specifically in how they consider characterizing their perovskite materials.
Sectors Electronics,Energy,Environment