Large-Area, Full-Color Image Sensors Made with Semiconducting Polymers

Abstract

The photoresponse of semiconducting polymers can be enhanced significantly by photoinduced charge transfer (CT), which separates electrons and holes and prevents early time recombination. [1±8] The discovery of photoinduced CT in composites of conducting polymers (as donors, Ds) and buckminsterfullerene, C 60 , and its derivatives (as acceptors, As) provided a molecular approach to high-efficiency solar cells and high-sensitivity photodetectors. [1] Because the time scale for photoinduced CT is of subpicosecond nature (faster than all competing processes), the quantum efficiency for charge separation from D to A is close to unity. By controlling the morphology of the phase separation into an interpenetrating bicontinuous network of D and A phases, one can achieve a high interfacial area within a bulk material: a ªbulk D/A heterojunctionº material that enables efficient charge separation. Such bicontinuous networks (with domain sizes of 5±50 nm) have clearly been demonstrated using a transmission electronic microscope (TEM). [8,9] The bicontinuous network provides the pathways needed to collect the separated carriers at the external electrodes, holes from the D phase and electrons from the A phase. Thus, thin-film sandwich devices with a bicontinuous D/A composite as the active material function as efficient photodetectors with high carrier collection efficiency. [4±8] High-performance photosensors in the metal/polymer/metal configuration have been demonstrated with quantum efficiency close to 100 % el/ph. [4±6] Because of the significant processing advantages of semiconducting polymers, these polymer photosensors can be fabricated by simple coating processes at low temperature. Consequently, the photosensor arrays can be produced on flexible substrates or on curved substrates. Moreover, these thin-film photodetectors can be used as pixel elements of large-area, full-color image sensors such as linear or two-dimensional (2D) digital cameras.We demonstrate, for the first time, large-area image sensors made with organic photodiodes. These image sensors have high photosensitivity, low dark current, and large dynamic range. Combining a polymer detector array with a set of optical filters, full-color image sensing in the visible spectral range is demonstrated. By proper selection of the sensing materials and/or optical filters, photosensors in other spectral ranges (e.g., UV or near-infrared) can also be achieved.For imaging applications in the visible portion of the spectrum, the spectral response of the sensor pixels should cover wavelengths between 400 and 700 nm. This can be achieved with a polymer blend containing poly(3-octyl thiophene) (P3OT) and PCBM [6,6], which is a derivative of C 60 with improved solubility. [5] The molecular structures of these components are shown at the top of Figure 1. P3OT was synthesized using the standard procedures with FeCl 3 as the catalyst. The polymer was then redissolved in chloroform and precipitated. Details of the synthesis of PCBM [6,6] and other C 60 derivatives were p...

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