DETECTORS: Near-IR imager uses quantum-dot-sensitized photodiodes

摘要

Indium gallium arsenide (InGaAs) photodiodes and quantum-well IR photodetectors are typical methods of choice for near-IR detection and imaging applications in the spectral region around 1 to 2 (mu)m (see www.laserfocusworld.com/articles/358147). But because these materials are expensive for large-area devices, researchers have been looking for organic-semiconductor alternatives. Unfortunately, polymers or small-molecule materials with low-enough bandgaps are not efficient as photodetectors in the spectral region above 1 (mu)m. An alternative, however, has been developed by researchers at Siemens Corporate Technology (Erlangen, Germany), the University of Linz (Linz, Austria), and Universitat Karlsruhe (Karlsruhe, Germany) that uses solution-processed hybrid diodes from colloidal quantum dots (QDs) and organic semiconductors deposited over a traditional amorphous silicon active-matrix (a-Si AM) backplane to fabricate a flat-panel near-IR imager. To form the organic/inorganic QD photodiode layer, a mixture of two organic semiconductors, P3HT and PCBM, were combined with lead sulfide (PbS) QDs with diameters ranging from 4.4 to 5.2 nm. Colloidal PbS QDs were chosen due to the effortless scalability of the synthesis and the applicability of the QDs as synthesized for hybrid diodes. With defined diameters for the QDs, the team demonstrated IR diodes with tunable spectral sensitivity up to 1.9 (mu)m. The near-IR photosensitivity is based on the concept of charge separation between an IR sensitizer (being the PbS QDs) and separate electron- or hole-accepting transporting materials (the PCBM and P3HT). An evaporated metal top contact, a polymer interlayer, a bottom electrode, and a 256 X 256-pixel a-Si AM backplane complete the detecting structure (see figure).