LAUSR

Organic nonvolatile memory devices have attracted increasing attention in both academia and industry due to their numerous advantages for application in charge storage media, including light weight, low cost, and flexibility.[1–3] Polymer electret memory (PEM) devices based on organic thin-film transistors (OTFTs) are an important type of organic nonvolatile memory devices in which memory arises from fieldeffect modulation via efficient charge injection in chargeable polymer gate electrets. The development of high-performance PEM devices has attracted considerable interest because of their simple fabrication process and good repeatability.[4–9] Previous studies have shown that the memory window depends strongly on the polarity, conjugation length, and architecture of polymer electrets.[10–12] For instance, photosensitive electret materials have been reported by Jeong et al.[13] and glucosebased oligosaccharides or polysaccharides as green materials have been reported by Chen’s group.[14] Also, novel blocking oxide materials have been investigated. Yeh et al.[15] reported low-operation-voltage (<20 V) memory devices based on highpermittivity (high-k) HfO2 blocking oxides and polymer electret layers. They found that the trapped charge begins to seriously degrade after only 103 s. Nam et al.[7] reported a memory device with a very low-operation voltage (5 V) based on the high-k polymer poly(vinyl alcohol), which was used as both a blocking oxide and a charge trapping layer. But the programming/ erasing (P/E) pulse width for this device was relatively long (3 s). Despite the significant progress in PEM devices,[10–18] simultaneously achieving high speed, high reliability, and lowoperation voltage in PEM devices still remains challenging. The underlying device physics related to P/E processes and charge trapping mechanisms remain unclear or under debate.[11,14,19–21] For instance, memory devices based on unipolar organic semiconductor pentacene as the active layer exhibit bipolar memory characteristics.[19] Li et al.[20] fabricated a poly(methyl methacrylate)–OTFT memory device with a p-type pentacene channel that only exhibited trapping and detrapping of minority electrons. Chiu[21] reported the trapping of minority holes in an n-type semiconductor-based memory Write-once-read-many (WORM) memory behavior is often observed in polymer electret memory (PEM) devices, greatly limiting their overall performance. This paper systematically investigates the device physics of PEM devices with poly(α-methylstyrene) as a charge trapping layer and pentacene as a semiconductor channel. The combined experiments on transistors, capacitances, and optical spectroscopy reveal that both the WORM memory behavior after negative and positive pulses and the gradual formation of memory after the continuous scanning are the results of the deficiency in minority (electrons) transport and trapping. Corresponding quantitative models are established and well explain the two-stage, gradual trapping processes to form memory. By reducing the structural disorder and lateral channel length, ambipolar, bistable memory and much faster formation of memory window is obtained based on the same PEM device. The insights into device physics of PEM devices are expected to facilitate the design of organic, nonvolatile memory devices with high programming and erasing efficiencies.