Organic Semiconductor Memory using Electronic Function of Photochromic Molecules

光致变色二芳基乙烯是公认的具有异构化热稳定性和抗疲劳性的材料。本文研究了一种新型的光致变色有机存储器件的原理。这种存储器件基于通过载流子注入的光致变色双稳分子的可逆异构化反应。这是通过载体(空穴和电子)遇到二芳基乙烯衍生物分子的激发来实现的。分子最低空轨道能级上的电子和最高占据分子轨道能级上的空穴产生与光激发相同的激发态，从而使分子转变为另一种异构化形式。对于二芳基乙烯衍生物，分子的电离势Ip随其异构化形式的不同而变化。闭环型(有色型)的Ip为5.7 eV，开环型(无色型)的Ip大于6.2 eV。信息的读出是通过检测分子间电流的差异来实现的。设计了一种新型的具有三苯胺基和恶二唑基的非对称双极性二芳基乙烯，分别具有电子供体和受体的特性。在实验中使用了薄膜样品来确认“写”和“读”原理。该存储器件由阳极、空穴注入层、空穴传输层、光致变色层和阴极组成。观察到可逆的电流下降，从而成功地证明了载流子注入的异构化反应。对于具有电子传输层的存储器件，信息写入速度显著提高。这归因于电子从阴极的有效注入和输运。为了实现更高速度的存储器件，需要开发双极性的二芳基乙烯衍生物。

Photochromic diarylethenes are well known materials which have thermal stability for both isomerization state and a fatigue resistance. In this study a novel principle of organic memory device using a photochromic molecule was studied. Such a memory device is based on a reversible isomerization reaction of a photochromic bistable molecule by an electric carrier injection. This is achieved by a carrier (hole and electron) encounter excitation of the molecule of a diarylethene derivative. The electron on lowest unoccupied molecular orbital level and the hole on highest occupied molecular orbital level of the molecule produce the excited state identical to that produced by photo excitation, so the molecule is transformed into another isomerization form. For the case of diarylethene derivatives, the ionization potential Ip of the molecule is changed according to its isomerization form. The closed-ring form (colored form) has Ip of 5.7 eV and the open-ring form (uncolored form) has above 6.2eV. The information readout is achieved by detecting the electric current through the molecule affected by the difference of Ip.A new nonsymmetrical bipolar diarylethene with a triphenylamine group and an oxadiazole group, which exhibited electron donor and acceptor characteristics, respectively, were designed. The thin film sample was used in the experiment to confirm the "writing" and "reading" principles. The memory device was consisted of an anode, a hole injection layer, a hole transport layer, a photochromic layer and cathode. Reversible electric current decrease was observed and, therefore, the isomerization reaction by carrier injection was successfully demonstrated.For the memory devices with an electron transport layer the information writing speed was considerably improved. This was attributed the effective electron injection and transport from the cathode. Development of the bipolar diarylethene derivative is required to achieve higher speed memory devices.