Researchers in-situ observe the ultrafast dynamics of trapped carriers in organic methyl ammonium lead halide perovskite thin films by ultrafast photocurrent spectroscopy

Researchers from Clemson University, Los Alamos National Laboratory, Huazhong University of Science and Technology, Jilin University, Kowloon Tong Hong Kong, the Israeli Technion and The University of Alabama have used laser spectroscopy in a photophysics experiment, and have broken new ground that could result in faster and cheaper energy to power electronics.

This novel approach, using solution-processed perovskites, could revolutionize a variety of everyday objects such as solar cells, LEDs, photodetectors for smart phones and computer chips. The goal of the research was to make materials that are more efficient, cheaper and easier to produce.

"Perovskite materials are designed for optical applications such as solar cells and LEDs," said Kanishka Kobbekaduwa, a graduate student at Clemson and first author of the research article. "It is important because it is much easier to synthesize compared to current silicon-based solar cells. This can be done by solution processing'whereas in silicon, you have to have different methods that are more expensive and time-consuming."

The unique method used by Clemson's Jianbo Gao's team'employing ultrafast photocurrent spectroscopy'allowed for a much higher time resolution than most methods, in order to define the physics of the trapped carriers. Here, the effort is measured in picoseconds, which are one trillionth of a second.

"We make devices using this (perovskite) material and we use a laser to shine light on it and excite the electrons within the material," Kobbekaduwa said. "And then by using an external electric field, we generate a photocurrent. By measuring that photocurrent, we can actually tell people the characteristics of this material. In our case, we defined the trapped states, which are defects in the material that will affect the current that we get."

The researchers can then identify the defects'which ultimately create inefficiency in the materials. When the defects are reduced or passivated, this can result in increased efficiency, which is critical for solar cells and other devices.

As materials are created through solution processes such as spin coating or inkjet printing, the likelihood of introducing defects increases. These low temperature processes are cheaper than ultra-high temperature methods that result in a pure material. But the tradeoff is more defects in the material. Striking a balance between the two techniques can mean higher-quality and more efficient devices at lower costs.

The substrate samples were tested by shooting a laser at the material to determine how the signal propagates through it. Using a laser to illuminate the samples and collect the current made the work possible and differentiated it from other experiments that do not employ the use of an electric field.

"By analyzing that current, we are able to see how the electrons moved and how they come out of a defect," said Adhikari of the UPQD group. "It is possible only because our technique involves ultrafast time scale and in-situ devices under an electrical field. Once the electron falls into the defect, those who experiment using other techniques cannot take that out. But we can take it out because we have the electric field. Electrons have charge under the electric field, and they can move from one place to another. We are able to analyze their transport from one point to another inside the material."

That transport and the effect of material defects upon it can impact the performance of those materials and the devices in which they are used. It is all part of the important discoveries that students are making under the guidance of their mentor, creating ripples that will lead to the next great breakthrough.

Posted: Mar 14,2021 by Roni Peleg