Organic Semiconductors - Yuuki Handoutai

Generally, it refers to organic compounds with electrical resistance smaller than 10 ⁻¹³ Ω cm. In the early 1950s, Hideo Akamatsu and Hiroo Iguchi began to study the electrical conductivity of a series of condensed polycyclic aromatic compounds. For example, they discovered that violanthrene had an electrical conductivity of 10 ⁻⁷ Ω ⁻¹ cm ⁻¹ , and this name was given to these compounds. Then, they found that a charge-transfer complex of perylene and iodine had a good electrical conductivity of about 10 ⁻¹ Ω ⁻¹ cm ⁻¹ , and research on this type of organic semiconductor became active. In 1964, WA Little published a paper on high-temperature superconductors, proposing a compound with a long chain of conjugated double bonds in the main chain and a polarizable dye as a side chain, suggesting that if such a compound could be synthesized, superconductivity might occur even at room temperature. This proposal strongly stimulated research on compounds with a chain-like structure in the charge-conducting part, which are called one-dimensional conductors in recent years. 7,7,8,8-Tetracyano-p-quinodimethane (TCNQ), developed by DuPont in 1960, is a planar compound with a conjugated system that bonds with other atoms or molecules to form salts with a linear structure, and these salts show good electrical conductivity. For example, single crystals of a 1:1 charge-transfer complex of TCNQ and tetrathiafulvalene (TTF) form linear columns in which the TCNQ and TTF molecules are stacked, and the electrical conductivity along these columns is 10 ³ Ω ⁻¹ cm ⁻¹ at room temperature, and further increases sharply at around 60 K, becoming comparable to the electrical conductivity of copper at room temperature. At first, this sudden increase in electrical conductivity was thought to be due to superconductivity, but this is now ruled out. Recently, it has been discovered that compounds with long conjugated systems such as polyacetylene and poly-p-phenylene can become p-type, with a conductivity of approximately 10 ³ Ω ^-1 cm ^-1 , by adding acceptors such as halogens or AsF ₅ , and can change from n-type semiconductors to metallic electrical conductors by adding donors such as alkali metals, and research into polymer semiconductors is also being actively conducted (see alternative terms: p-type semiconductor, n-type semiconductor). On the other hand, polymers such as poly( N -vinylcarbazole) are complete insulators in the dark, but exhibit high photoconductivity when exposed to light (especially ultraviolet light). Many organic compounds that exhibit this type of photoconductivity are also known, and some are used in practical applications such as electronic copying.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

一般に，電気抵抗が 10^－13 Ω cm より小さい有機化合物をいう．1950年代のはじめに，赤松秀雄と井口洋夫が一連の縮合多環芳香族化合物についての電気伝導性の研究をはじめたが，たとえばビオラントレンが 10^－7 Ω^－1 cm^－1 の電気伝導率を示すことを発見して以来，これらの化合物に与えた名称である．ついでペリレンとヨウ素との電荷移動錯体が約 10^－1 Ω^－1 cm^－1 の良好な電気伝導率を示すことを見つけ，この種の有機半導体の研究が活発に行われるようになった．1964年には，W.A. Littleが高温超伝導体に関する論文を発表し，そのなかで共役系の二重結合が長く連なった主鎖に分極性の色素を側鎖としてもつ化合物を提案し，このような化合物が合成できたら室温でも超伝導現象が現れるかもしれないことを示唆した．この提唱は，最近，一次元伝導体などとよばれている電荷を導く部分が連鎖状の構造をもつ化合物に対する研究に強い刺激となった．1960年にデュポン(DuPont)社で開発された7,7,8,8-テトラシアノ-p-キノジメタン(TCNQ)は，共役系をもつ平面上の化合物で，ほかの原子あるいは分子と結合して直鎖状構造をもつ塩を形成し，これらの塩は良好な電気伝導性を示す．たとえば，TCNQとテトラチアフルバレン(TTF)との1：1の電荷移動錯体の単結晶は，TCNQとTTFの分子がそれぞれ積み重なった直鎖状のカラムを形成し，このカラムにそった電気伝導率は室温で 10³ Ω^－1 cm^－1 であり，さらに60 K 付近で急上昇し，銅の室温での電気伝導率に匹敵するようになる．最初，この電気伝導率の急上昇は超伝導ではないかと考えられたが，現在では否定されている．最近では，ポリアセチレンやポリ-p-フェニレンのような長い共役系を有する化合物も，ハロゲン，AsF₅などのアクセプターを添加することにより，10³ Ω^－1 cm^－1 程度までの伝導性を有するp型の，またアルカリ金属などのドナーを添加することによりn型の半導体から金属的電気伝導体となることが知られ，高分子半導体の研究も活発に行われている([別用語参照]p型半導体，n型半導体)．一方，ポリ(N-ビニルカルバゾール)のようなポリマーは，暗所では完全な絶縁体であるが，光(とくに紫外線)を照射することにより大きな光伝導性を示す．このような光伝導性を示す有機化合物も多く知られ，電子複写などに実用されているものもある．

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