Digital x-ray imaging offers an attractive alternative for applications in medical radiography. It has all the advantages of digital cameras over film cameras — including instant image reproduction and transmission, high sensitivity and a greater dynamic range, which helps to limit under- and overexposure of images. In addition, the imaging technique enables computer-aided processing and digital archiving, while higher-quality images can be acquired with a lower dose of radiation compared with conventional x-ray imaging.Researchers have demonstrated the efficacy of polymer-based photodiodes for digital x-ray imaging. Use of organic materials could contribute to lower production costs of imaging arrays and thus could help to introduce digital x-ray imaging to the clinic. A polymer-based photodiode is shown here. PEDOT:PSS is the most common material in organic electronics and works as the anode of the photodiode; ITO represents the indium tin oxide layer.Currently, there are several digital x-ray imagers on the market. These are based on conventional semiconductors such as amorphous silicon, however, and for this reason are expensive to produce. The imaging arrays might therefore benefit from the use of organic semiconducting polymers, which has been described for large-area applications, including solar cells and flat panel displays. Organic electronics can be fabricated with relatively inexpensive techniques and are flexible, facilitating curved or “bendy” imaging arrays.Organic electronicsIn the Jan. 16 issue of Applied Physics Letters, researchers in the UK with Cavendish Laboratory, in Cambridge, with University College London and with Imperial College, also in London, reported a study in which they tested the efficacy of organic electronics — made of conjugated polymers — for digital x-ray imaging. These materials could help to lower the production costs of imaging arrays and thus facilitate widespread clinical implementation of this method.“Conjugated polymers are considered interesting alternative electrophotonic materials for the development of optoelectronic devices,” said Panagiotis E. Keivanidis, an author of the study. “The optimized chemical protocols for the synthesis of conjugated polymers have increased their chemical purity. Moreover, their electronic properties can be easily tuned via simple chemistry steps.”Also, because conjugated polymers are soluble in most organic solvents, investigators can achieve large-area deposition using inexpensive techniques such as spin coating and jet printing. “These deposition techniques sound particularly attractive for the deposition of polymer-based electroactive thin films onto flexible plastic substrates,” Keivanidis said.The researchers assessed the x-ray hardness of organic photodiodes based on three donor-acceptor blend systems. To distinguish the effect of ambient aging from the impact of the x-rays on the external quantum efficiency and on dark current density-voltage curves, they prepared reference and active samples and characterized them both immediately after fabrication, before the exposure of the active samples to x-rays. Both reference and active samples were characterized again within four weeks after the x-ray exposure.They found that polymers did not degrade significantly after large doses of radiation — equivalent to the lifetime doses used in medical x-ray imaging. “Before this study was carried out, there was a concern that such semiconductors might degrade when exposed to x-rays, as this is sometimes a problem when similar materials are exposed to UV radiation, such as in sunlight,” said James C. Blakesley, another author. “Clearly, it is important that the polymers do not stop working when they are exposed to clinical x-rays.”In addition, they demonstrated that polymer diodes could detect x-rays when coupled to gadolinium oxysulfide or to other x-ray-to-light-converting materials. “This is an important first step in showing that polymer electronics could be used for this application.”Still, much remains to be done before polymer photodiodes can be implemented clinically for digital x-ray imaging. For example, such photodiodes typically exhibit either a high dark current or a low sensitivity to light — the former resulting in noisy digital images and the latter, in underexposed images. The photodiodes must be optimized to have a low dark current as well as a high sensitivity, to minimize the amount of radiation needed to yield a good image. Then, technology must be developed to fabricate arrays of organic photodiodes and to integrate them with the transistor arrays that enable digital readout.The researchers are working toward both of these. They are seeking a way to reduce the dark currents in photodiodes with multilayer structures. Also, they soon will evaluate the performance of a single-pixel detector composed of a polymer photodiode integrated with an organic thin-film transistor. “This is the basic building block that needs to be multiplied by several million to make a useful imaging array,” Blakesley said.Contact: Panagiotis E. Keivanidis, Imperial College, London; e-mail: pekeivan@imperial.ac.uk.