(Invited) Polymer Resistive Electronic Memory: Exploiting Bistability of Current-Voltage Characteristics
Authors: Pfleger Jiri , Panthi Yadu Ram , Rais David , Výprachtický Drahomír 
Published: February 2020

Electronic memory elements represent indispensable components of most electronic circuits. For some applications, like RFID tags used for smart packaging, for manufacturing and warehouse automation or security systems, simple low-level performance devices are required, which could be produced by cheap large scale production technology, as e.g. printing. We focus on resistor-type memory (ReRAM) made from switchable resistive materials. Unlike transistor and capacitor memory devices, ReRAM does not require a specific cell structure, which is difficult to prepare by printing. ReRAM is a simple two electrode device based on a sandwich structure, in which data are stored as different electrical conductivity states. Electrical bistability usually arises from changes of the intrinsic materials properties, such as charge transfer, phase change, conformation change and reduction–oxidation (redox) reaction, in response to an applied electric field. The basic memory cells can be integrated into a cross-point memory array, where each memory cell is unambiguously identified by its x and y coordinates, suitable for easy programming and reading the addressed cell.
We show the preparation and properties of simple organic memory elements based on thin films of soluble polymers, which can be prepared using common laboratory casting techniques as spin casting, slot die or dip coating, or screen and inkjet printing in the manufacturing scale. Two systems are presented:
(i) Molecularly-doped polymer based on a soluble derivative of polydiketopyrrolopyrrole mixed with a soluble low-molecular weight derivative of perylene sandwiched between ITO and Al electrodes shows pronounced hysteresis in its current voltage characteristics. Depending on the composition, the maximum achieved ON/OFF ratio was about 5 with writing/erasing voltage +/-5 V and reading voltage 0.5 V. The functionality of the structure as rewritable resistive memory is well documented on repeated read-write cycles and a retention time exceeding several hours. The properties of the memory element were further improved by the incorporation of a very thin insulating layer of high-k dielectrics containing molecular groups with high permanent dipole. Such elements reached the ON/OFF ratio about 103.
(ii) A single component polymer rewritable resistive memory was prepared using a newly synthesized poly(N-(3-(9H-carbazol-9-yl)propyl)methacrylamide) sandwiched as a thin film between ITO and Al electrode. The polymer was prepared by a radical polymerization of the N-(3-(9H-carbazol-9-yl)propyl)methacrylamide using azobisisobutyronitrile as a radical initiator. The starting monomer was obtained by three-step reaction from carbazole. The molar mass of the polymer was influenced in limited extent by the reaction temperature but it was still relatively low (M
w = 6000, M
n = 4000, Ð = 1.50). However, the film forming abilities were sufficient, particularly in thin films (typical thickness used in the memory elements was below 100 nm). The polymer is stable at higher temperatures up to 300 °C, glass transition temperature of the polymer is 130 °C.
The device showed good hysteresis in I-V characteristics with switching effects, maximum ON/OFF ratio ~ 200..A typical write/erase cycle is shown in Fig. 1 showing a good retention time exceeding 2 hours even during permanent reading in both ON and OFF states. Here, writing was at the applied voltage +5 V erasing with -5 V and reading at 0.5 V (sign refers to the polarity of the ITO electrode). After some initial instability during memory element forming, the ON and OFF current was stable during more than 40 repeated write/erase cycles.
Work supported by the Grant Agency of the Czech Republic, project No.17-03984S

Figure 1