Fullerene Photodetectors with a Linear Dynamic Range of 90 dB Enabled by a Cross-Linkable Buffer Layer

Organic photodetectors have attracted signifi cant attention for their immediate application to the replacement of conventional devices based on expensive inorganic semiconductors. Potential new uses in both civil and defense applications take advantage of organic photodetectors’ light weight, fl exibility, and excellent form factors. [ 1 ] An example of a unique application for an organic photodetector is the capability to form organic thin fi lm devices on curved surfaces for things such as an artifi cial eye and a hemispherical array detector for imaging (HARDI), which allows a very wide fi eld of view without the aberrations encountered in a fl at focal plane. [ 2 ] In addition to the low cost of materials and the fabrication process, which is the typical advantage of organic electronic devices, organic photodetectors have the potential to outperform their inorganic counterparts; because their low conductivity enables them to dramatically reduce noise. A recent study shows that polymer-based organic photodetectors can have higher sensitivity than traditional photo detectors, such as silicon and InGaAs photodetectors in some wavelength range. [ 3 ] The response characteristics, such as linearity, to very weak light are of ultimate importance when detectivity of organic photodetectors reaches the parity with traditional photodetectors. Recent studies on organic photodetectors have focused on improving detectivity, but little attention was paid to the linearity of the organic photodetector’s responsivity, especially in a low light intensity region. The reported detectivities were mostly calculated according to the organic photodetector’s reponsivity at relatively strong light levels: orders of magnitude larger than the calculated noise equivalent power (NEP). It has not been shown yet whether the organic photodetectors can still maintain the high responsivity at low incident light intensity close to the NEP. However, there is concern that the organic photodetector would loss its high responsivity at such a low light level as there is generally a much higher density of charge traps in organic rather than inorganic semiconductor materials due to the amorphous or polycrystalline organic semiconductors used. When the charge density generated by the incident light is comparable to the charge trap density, the photogenerated charges might be trapped rather than contribute to the device photocurrent. In this paper, we report on a highly sensitive, fullerene-based organic photodetector device which shows linear response from the indoor light intensity all the way down to 12 pW m 2 . This type of organic photodetector presents a linear dynamic range of 90 dB. Our fi rst step is to fabricate a photodetector with very small noise current so that the weak light generated small photocurrent can be distinguishable from the noise current. The lightabsorbing material used in this study is fullerene (C 60 ) which is one of the most broadly studied materials for devices, including organic solar cells and organic fi eld effect transistors, because of its excellent optoelectronic properties, such as a large light absorption coeffi cient, of 1.21 ◊ 10 5 cm 1 at 442 nm, [ 4 ] and high electron mobility, of up to 6~11 cm 2 V 1 s 1 . [ 5 ] In our organic photodetector devices, the thickness of C 60 is around 80 nm which allows more than 60% of the light above its optical bandgap to be absorbed. In our previous study, it was found that C 60 is a good photoconductor material with much longer hole trapping time than electron transit time. A photoconductive gain above 50 under reverse bias below −5 V has been observed in the device with a structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)