Flexible Solar Cell Integrated with Memory
Professor Kim Tae-geun’s group developed a 2D MXene-based memristive organic solar cell with excellent mechanical stability.

The research results were published in Advanced Science, an internationally renowned journal.

▲ (From left) Professor Kim Tae-geun of the School of Electrical Engineering (corresponding author), Kiran A. Nirmal (Ph. D student, first author), and Wanqi Ren (Ph. D student, first author).

The research group led by Professor Kim Tae-geun of the School of Electrical Engineering of the College of Engineering successfully developed a memristive organic solar cell (MemOSC) that integrates the memory and learning functions in a single chip by using a 2D titanium carbide (Ti3C2Tx) MXene-based multilayer (Ti3C2Tx MXene/Ag/ Ti3C2Tx MXene) flexible transparent electrode.
* MXene: A ceramic material with a 2D planar structure. MXene includes layers with the thickness of atoms in which carbon or nitrogen is bound to a transition metal. Since it has conductivity as well as hydrophilicity due to a terminal hydroxy group or oxygen atom, MXene is solution processible.

Organic solar cells (OSC) have gained much attention due to their low production cost, low weight, mechanical flexibility, and ease of large-area fabrication. Studies have been actively conducted to grant learning capabilities to the devices. Meanwhile, developing an electrode that has low sheet resistance, transparency, and high mechanical stability is essential for improving the performance of all photovoltaic devices, including solar cells. Currently, conventional indium tin oxide (ITO)-based electrodes are used commercially due to their high transmittance and high conductivity, but they have the drawbacks of low mechanical flexibility and the high price of the constituent indium.

The research group doped Zn nanoparticles onto the surface of an electrode with a Ti3C2Tx MXene/Ag/Ti3C2Tx MXene multilayer structure fabricated by spin coating and sputtering. In this way, the group successfully developed a flexible transparent electrode featuring a low sheet resistance of 9.7 Ω sq-1, a high transmittance of 84%, stable properties even after a 2,000-cycle bending test, and an easily controlled electrode work function. The memristive organic solar cell (MemOSC) equipped with the developed electrode exhibited a power conversion efficiency of 13.86% and sustained stable photovoltaic performance even after hundreds of switching cycles. The MemOSC also demonstrated excellent memory performance characterized by a low operating voltage of 0.06 and -0.33 V, a high ON/OFF ratio (103), stable endurance performance (4x103), and excellent memory retention properties (>104 s).
* Spin coating: A method of coating a liquid onto a substrate by using the centrifugal force generated by rotating the substrate. The method allows the fabrication of a uniform thin film.
* Sputtering: A method of forming a film on a substrate by filling a vacuum container with an inert gas such as argon (Ar), discharging electricity, and accelerating ionized gas to collide with a target so that atoms or molecules detach from the target.
* Flexible transparent electrode: A thin film featuring flexibility as well as high transmittance and conductivity. The film is an essential material for determining the performance of photovoltaic devices such as flexible solar cells and displays.

The results showed that the 2D MXene-based multilayer electrode structure functions well as the flexible transparent electrode of OSCs and that it can function as electrically readable nonvolatile memory. In addition, energy harvesting and artificial synaptic (memory and learning) functions may be integrated into the structure in the future to produce integrated energy devices and systems.

The research was supported by the Research Leader Program funded by the Ministry of Science and ICT and the National Research Foundation of Korea (NRF). The article was published in Advanced Science [IF:17.521, JCR, Journal Citation Reports: top 5.94%], a globally renowned journal, on May 3, 2023.
* Title of Article: Flexible Memristive Organic Solar Cell using Multilayer 2D Titanium Carbide MXene Electrodes
* Name of Journal: Advanced Science (Adv. Sci. 2023, https://doi.org/10.1002/advs.202300433)



[ Description of figures ] 

▲Figure 1. A schematic diagram of the hybridization of the MXene sheet and Zn nanoparticles (left); and the structure of the MemOSC device fabricated using the developed Ti3C2Tx MXene/Ag/ Ti3C2Tx MXene-based flexible and transparent electrode and the results of the current density-voltage measurement of the device before and after Zn doping (right).



▲Figure 2. The current-voltage switching characteristics of the MemOSC (left) and the results of the anti-symmetric Hebbian learning rules mimicked by the MemOSC (right).