High performance and low cost silicon-based non-volatile memories—such as flash-RAM—have rendered floppy discs and many other forms of portable storage obsolete. But for uses that require data to be written only once, such as archiving and security applications, the cost effectiveness of the silicon devices is limited. So-called write-once-read-many—WORM memories— made from low-cost organic materials could be an alternative approach to overcome this problem.

Now, Moonhor Ree and colleagues1 from the Pohang University of Science & Technology in Korea have developed devices based on copper phthalocyanine (CuPc)—a widely used organic semiconductor. This material is usually grown by vacuum deposition techniques and does not usually exhibit switching behaviour that could be used for data storage. However, when the authors grew thin polymer films with hyper-branched microscopic structures from solution, they found novel switching properties with potential applications for memory devices.

Fig. 1: Schematic illustration of the authors WORM memories. A hyperbranched copper phthalocyanine (HCuPc) is grown from solution on an indium-tin-oxide (ITO) layer, and sandwiched beneath gold contact patterned on top. By applying a bias between the two conducting contacts, the state of the HCuPc material between them can be switched or read.

Specifically, they discovered that when a voltage of just over 2.5 V was applied across such a film sandwiched between indium-tin-oxide and gold contacts (Fig. 1), the CuPc films switched from a high (ON) to low (OFF) conductivity state. The conductivity of the OFF state was more than one million times smaller than the ON state, making it easy to read the state of devices. And once switched to the OFF state they found the devices remained permanently in that state, even when tested again 12 months later.

The simplicity of the devices and the fact that the films on which they are based can be grown from solution, not only makes them potentially much cheaper than silicon-based memories, it could enable them to store much larger amounts of data.

“The density of the data that can be stored in silicon-based memories can only be improved by making the size of individual memory cells smaller. This is because they can only be fabricated in two-dimensions on the surface of a silicon chip,” says Ree. “In contrast, because we grow our films from solution, it should be possible to build 3D arrays of devices by spin-coating or dip-coating multiple layers, to achieve very high storage densities.”