Mimicing the Eye: Imagers Based on Hemispherical Focal Plane Arrays Using Organic Photodiodes

Stephen Forrest

The human eye provides an example of an ideal imaging system: it is compact and lightweight while having a very wide field of view without image distortion, a very low f/number (and hence high sensitivity in low light), and has a very simple lens system. The architecture is made particularly simple since the image is formed on a nearly hemspherical surface, thereby matching the curved focal plane of the lens. Achieving this imaging system in modern cameras has been difficult since the “film plane” must be flat if conventional, brittle semiconductor sensor arrays are used. Indeed, formation of high performance organic electronic devices on three dimensionally deformed surfaces is severely constrained by the tensile stresses and shear slip that are introduced during the deformation process. Here, we demonstrate the direct transfer of metals via cold welding onto preformed, 1.0 cm radius plastic hemispheres with micrometer scale feature resolutions to realize 100x100 organic photodetector focal plane arrays that mimic the architecture of the human eye [1]. This demonstration significantly extends the ability of direct transfer patterning, previously only demonstrated on planar substrates, to advanced optical and electronic applications. The passive matrix focal plane array consists of high performance, (40 μm)² organic double heterojunction photodetectors with response extending across the visible spectrum. The dark current density of a typical detector is 2.5±0.1 μA/cm² at -1V bias, and with a peak external quantum efficiency reaching 12.6±0.3% at a wavelength of 640 nm. The photodetector impulse response was 20 ns, making the array suitable for video recording applications.

[1] “Direct Transfer Patterning on Three Dimensionally Deformed Surfaces at Micrometer Resolutions and Its Application to Hemispherical Focal Plane Detector Arrays”, X. Xu, M. Davanco, X.Qi and S. R. Forrest, Org. Electron., 9 (2008).