High-detectivity printed organic photodiodes for large area flexible imagers

The recent increase in utilization of image sensors and significant development in the field of wearable and disposable sensors not only demands a decrease in photodiode fabrication costs, but also requires new functional abilities such as operating at extremely low light intensities, narrowband and broadband spectral selectivity, lightweight and mechanical flexibility. Today, most of these demands cannot be fulfilled by conventional workhorse silicon photodetectors without a significant amount of processing steps to achieve heterogeneous integration. The chemical tunability, mechanical properties and printable processing of organic materials make them promising candidates for fabrication of low-cost, highly scalable and flexible organic photodiodes (OPDs). In addition, the large area scalability of printing techniques is beneficial for photodiodes since the signal-tonoise ratio (SNR), at a given light intensity, increases with detector size. Since the SNR depends on multiple parameters, such as the external quantum efficiency (EQE), the dark current density, sampling bandwidth and photoactive area, it significantly complicates the comparison of photodetectors. For this reason a useful figure of merit to consider is specific detectivity. This paper shows that state of the art all-printed, flexible OPDs with average specific detectivities as high as 3.45×1013cm-Hz0.5·W-1can be printed using industrially scalable techniques (i.e., blade coating and screen printing) by controlling charge selectivity of organic electrodes. In addition to the high specific detectivity needed to resolve low light intensity, these OPDs are able to sustain high reverse biases. High reverse biases are used to maximize the capacity of the photodiode to capacitatively store photogenerated charge, which is essential for increasing the dynamic range of image sensors. Through optimization and characterization of the properties of each printed layer, device yield is maximized with minimal variability over large areas. The performance of all-printed photodiodes achieved here becomes comparable to silicon photodiodes used in image sensors, which have a specific detectivity of approximately 6-7 × 1013cm-Hz0.5·W-1. These results show that along with advantages such as inexpensive fabrication and mechanical flexibility, all-printed OPDs can have performance competitive with their Si-counterparts.