使用有机半导体的光电器件因其轻薄、灵活性等独特的特点而备受关注。通常,半导体器件由以硅为代表的无机材料组成。一方面,由于无机材料的刚性,用其制造“柔性”器件基本上是困难的。另一方面,有机半导体可以在 100 nm 的薄膜中工作,并且可以通过印刷或蒸发等普通处理技术在塑料或超薄玻璃薄膜等柔性基板上形成。针对有机光电器件,我们已研究作为光电传感器的有机光电二极管 (OPD) 和作为光源的有机发光二极管 (OLED)。
图 1:有机光电器件
图 2:OPD 和 OLED 的工作机理
有机光电器件的基本结构和工作机理如下。OPD 和 OLED 的结构都很简单,其中有机半导体夹在阳极和阴极之间。在 OPD 中,有机层包括电子供体分子和受体分子。当这些分子吸收光子时,会产生静电耦合的电子空穴对,称为激子。激子扩散到供体-受体接口,并解离成自由载体。最后,在每个电极处收集孔和电子,并将其检测为电信号。由于有机半导体在本质上具有与绝缘体相似的特性,因此有机层需要非常薄。这种结构表明,OPD 沿垂直(厚度)方向具有导电性,而沿水平轴方向具有高电阻。这进而表明,与传统的无机器件相比,几乎没有观察到电串扰。此外,有机半导体在薄膜中显示出很高的光电变换效率,因为光吸收系数较高。此特性也支持有机材料用作薄器件的适用性。
另一方面,在 OLED 中,从每个电极注入的孔和电子在发光的有机分子中再结合,并且观察到光发射。借助使用透明电极和简单制造工艺的夹层结构,OLED 适用于大面积表面发射光源。这可以实现高照明均匀度和优选的图案设计。
我们已开发在近红外 (NIR) 区域具有感光灵敏度的柔性 OPD。800 nm 左右的波长范围被称为“生物光学窗口”,因为水或血红蛋白 (Hb) 等生物成分在 NIR 区域的吸光度较低。因此,光的散射比吸收更明显,我们可以获得有关活体的各种信息。我们的目标是将柔性 OPD 应用于生物传感设备,以降低测量中的压力。
图 3 (a):柔性 OPD
图 3 (b):光谱灵敏度特性
图 4:生物光学窗口
我们还展示了具有任意发射模式的大面积 NIR-OLED。我们的 NIR-OLED 即使在高电流密度下工作也具有极高的运行稳定性,此外,辐射功率仅 1 mW。利用表面发射器件,我们可以为传感或暗场观测应用提供能够均匀照射目标的光源,其前景非常广阔。
■我们的 OLED 的 NIR 图像
图 5:发射光谱
图 6 (a):传统 LED
图 6 (b):表面发射 OLED
*T. Yamanaka, H. Nakanotani, S. Hara, T. Hirohata, C. Adachi, “Near-infrared organic light-emitting diodes for biosensing with high operating stability,” Appl. Phys. Express 10, 074101 (2017).
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