Wednesday, July 01, 2009

Organic Photodiodes Improving

EETimes: Researchers from Siemens Corporate Technology (CT) have developed a cost-effective production process for organic large-area photo diodes: Instead of the usual method of casting or doctor blading, they succeeded in spraying the material.

The Siemens researchers claim their spraying method generates active semiconductor of a rather good quality. They say that the diodes feature very low dark current and also a very high efficiency of 75%. In addition, the lifetime can be expected at six years, and the speed is high enough to use the diodes in computer tomography applications. The image lag, another relevant parameter for these applications, is significantly better than with available photo detectors made of amorphous silicon.

The prototype introduced by the Siemens researchers features 256 x 256 pixels, but since the process scales well, it is said to be possible to produce diode arrays with more than 1000 x 1000 pixels.

Meanwhile, Fujifilm researchers present their organic sensor paper on Technical Group on Information Sensing Technologies (IST) meeting on July 24, 2009 in Japan. Their paper is titled:

"CMOS Image Sensor with a Thin Overlaid Panchromatic Organic Photoconductive Layer as the Best Candidate for Sensors with Reduced Pixel Size."

5 comments:

  1. I guess this announcement is related to their paper at Nano letters from march:
    http://pubs.acs.org/doi/abs/10.1021/nl803386y

    In case the link is not visible without subscription, the abstract reads:
    "Solution-processed organic diodes based on bulk heterojunctions are attractive for large area photodetection. We report a general approach for fully spray-coated organic photodiodes with outstanding characteristics in comparison to bladed or spin-coated devices. Despite the high surface roughness and the less defined morphology of the spray-deposited organic layers, we observe organic photodetectors with responsivities of 0.36 A/W and noise equivalent powers of 0.2 pW/H^(1/2) in the visible spectrum at high reverse biases of −5 V. Furthermore, we demonstrate device lifetimes beyond 1 year as well as superior yield and reproducibilties for the dark current and photocurrent densities."

    They seem to be doing nice things with organic photodiodes:
    http://w1.siemens.com/innovation/en/news_events/ct_pressreleases/e_research_news/2009/e_22_resnews_0911_1.htm
    (paper in nature photonics: http://www.nature.com/nphoton/journal/v3/n6/abs/nphoton.2009.72.html)

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  2. Lag? Dark current?

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  3. David,

    Thank you for the interesting links! If you happen to have access to these publications, what is their dark current and image lag? Also, do they tell something about reproducibility of their spray-based technology?

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  4. From the nano letters paper:
    "[...] Here, we report fully spray-coated OPDs
    with reproducible low dark current densities of ~ 65 nA/cm2 with EQEs as high as 76% at -5 V reverse bias."

    "[...] Continuous HC films are obtained with a layer thickness as thin as ~100 nm. The root mean square (rms) roughness value for a single spray-coated HC layer with an average thickness of ~220 nm is ~33 nm"
    HC being the "Hole Conductor" layer.

    "[...] the 25-75 percentile range of the dark current from 6.2 × 10-5 to 7.9 × 10-5 mA/cm2 and photocurrents from 0.232 to 0.237 mA/cm2. [...] Furthermore, transient measurements of spray-coated OPDs with an active area of 1 mm2 exhibit a -3 dB cutoff frequency at 100 kHz."

    They mention at the end of the paper that position-sensitive detectors using this technology will be reported "elsewhere".
    I will see if I can find it.

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  5. Thank you for the info. 65nA/cm2 is not that bad already. It translates to less than 1000e/s for 1.4um pixel, which is not that far away already. I wonder at what temperature this was measured?

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