半導体デバイスはシリコンに代表されるような無機固体材料によって構成されているものが一般的ですが、近年有機材料を半導体として用いたデバイスが注目を集めています。有機半導体材料は印刷や蒸着といったシンプルなプロセスで成膜することが可能であり、0.1ミクロンオーダーの薄膜で機能するため、フィルムや極薄ガラス上に素子形成を行うことで、薄い、軽い、曲げられるといった自由度の高いデバイスを実現することが可能です。
当社では、センサとして用いる有機受光素子(organic photodiode: OPD)などの光学デバイスの研究を行っています。
図1. 有機半導体デバイス
有機デバイスの基本的な構造は、有機半導体材料を電極で挟み込んだサンドイッチ構造となっています。OPDは電子供与性(ドナー型)、電子受容性(アクセプター型)の2種の有機分子が含まれており、それらの分子に光が吸収された後、ドナー-アクセプター分子界面に励起状態が移動することで電荷分離が誘起されます。その後、生成したホールと電子がそれぞれの電極に捕集されることで電流が発生し、シグナルとして検出されます。本来有機材料は絶縁体に近い特性を持っていますが、薄膜化することで半導体としての機能させています。そのため素子の垂直方向に対しては導電可能である一方で、水平方向に対しては高抵抗となります。つまり、一般的な無機半導体を用いた受光デバイスと比較して、電気的なクロストークが本質的に起こりにくいデバイスであるということが言えます。さらに光吸収係数が非常に高いために、0.1ミクロンという薄膜でも高い光電変換効率を得ることができ、これも薄膜デバイスとしての性能を支持する特長となっています。
図2. OPDの動作原理
当社で開発したフレキシブルOPDは近赤外領域に受光感度を有しており、生体センシング用デバイスへの応用を目指しています。この波長帯は水やヘモグロビンといった生体を構成する要素の吸収が小さく、光の振る舞いとしては散乱現象が支配的となるため「生体の窓」と呼ばれ、生体内の多くの情報を得ることが可能です。そのセンサ部にフレキシブルOPDを用いることで、装着感のない、生体にフィットする新しいデバイスが創製できると考えています。
図3(a)フレキシブルOPD
図3(b)受光感度
図4. 生体の光学的窓
当社では有機半導体分子のもつユニークな光特性にも着目し、研究を行っています。例えば、OLED分野などで精力的に研究されている発光性の有機分子において、光を照射することによって膜中に電荷が発生する、つまり発光材料であるにもかかわらず受光材料としての特性を併せもつことが明らかとなっています(参考文献2、3、4)。生成した電荷は、有機分子によって自発的に形成された分極電場によって長期間安定に保持され、外部電界の印加により再び光として取り出すことが可能です。さらに、生成電荷は空間情報やその電荷量までも保持することが可能であり、1つの有機分子において、受光、記憶、発光の3つの機能を兼ね備えた新奇有機光メモリデバイスとして機能させることが可能となります。図7は、任意のパターン光を照射することによってデバイス内部に電荷を生成し、一定時間経過後に外部電圧を印加したデモンストレーションです。本デバイスにおいて、特別な画素構造などは作り込まれていないにもかかわらず、空間情報および強度情報が保持された発光像が得られています。
図7.有機光デバイスのデモンストレーション
1. T. Yamanaka, H. Nakanotani and C. Adachi, Slow recombination of spontaneously dissociated organic fluorophore excitons. Nat. Commun. 10, 5748 (2019).
2. T. Yamanaka, H. Nakanotani and C. Adachi, Significant role of spin-triplet state for exciton dissociation in organic solids. Sci. Adv. 8, eabj9188 (2022).
3. T. Yamanaka, H. Nakanotani, K. Nakamoto and C. Adachi, Electron lifetime of over one month in disordered organic solid-state films. Adv. Mater. 35, 2210335 (2023).
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