Fig. 1: The transition between the OFF (low conductivity, left) state and the ON (high conductivity, right) state of the PVK–PF polymer.Reproduced with permission. Copyright 2008 American Chemical Society

Organic semiconductor polymers are of increasing interest because, compared with the usual solid-state kind, they are lightweight, easily shaped and can therefore be used to make flexible opto-electronic devices such as light-emitting diodes, transistors and lasers. These polymers transport charge via their orbitals, which can be in one of two configurations: either in series (conjugated) or stacked on top of each other. Poly(N-vinylcarbazole) (PVK) is a commonly used supramolecular organic semiconductor polymer, which transports charge via intrachain stacked —orbitals that result from face-to-face conformation of the carbazole groups. But PVK only has this ON state, and so far little work has been done in attempting to tune this charge-carrying behaviour. Now, Wei Huang and colleagues at Nanjing University of Posts and Telecommunications, Nanjing, China, and Fudan University, Shanghai, China have developed a method of functionalizing PVK by using a Friedel-Crafts reaction to add the bulky non-planar phenylfluorene (PF) as a side group, with the aim of using its steric effects to tune the charge transport of the resulting polymer1. The researchers created a PVK-PF sandwich device, and on sweeping a voltage across it from 0 to +3 V, measured a sharp increase of current at 2.2 V—effectively a transition to an ON state (10-7 A) from the low-conductivity OFF state (10-11A), which can be regarded as a ‘write’ process. On applying a reverse voltage sweep, the field effects induced conformational changes of the PF groups, which blocked the face-to-face orientation of the carbazole group—thus ‘erasing’ and forming the OFF state (Fig. 1). The researchers were able to repeat the write/erase process in PVK-PF with no degradation of the conductivity in the ON/OFF states after 108 read cycles, and after 6 hours of continuous stress tests.

The non-volatility and the erase process of the PVK-PF system is additional functionality to that of PVK alone, and demonstrates the first non-volatile flash-memory device that is based on conformational changes in the polymer system.