Large-area organic photodiodes can now offer advantages over conventional silicon photodiodes, particularly noise advantages, according to Georgia Tech, which has solution-processed flexible organic devices in arbitrarily shapes.

Georgia-Tech-organic-semiconductor
It claims performance comparable to that of rigid silicon photodiodes in the visible light spectrum, except their response time.
“What we have achieved is the first demonstration that these devices, produced from solution at low temperatures, can detect as little as a few hundred thousand photons of visible light every second, similar to the magnitude of light reaching our eye from a single star in a dark sky,” said Gatech researcher Canek Fuentes-Hernandez. “The ability to coat these materials onto large-area substrates with arbitrary shapes means that flexible organic photodiodes now offer some clear advantages over state-of-the-art silicon photodiodes in applications requiring response times in the range of tens of microseconds.”
The photodiodes use polyethylenimine surface modifier to produce air-stable low work-function electrodes, which also produces devices with low of dark currents. “Dark current levels were reduced so much that measurement equipment had to be redesigned to detect an electronic noise corresponding to a fluctuation of one electron in one millionth of a second,” said Fuentes-Hernandez.
Noise current in the tens of femtoampere range is claimed, as is noise equivalent power of a couple of hundred femtowatt.
Potential applications, according to the university, include pulse oximeters where 10x sensitivity might allow measurements in places other than fingertips, as receivers for x-rays, and in scintillation radiation detectors where a flash of light emitted by a phosphor when struck by a high energy particle is detected.
“We are working on improving the response time of the photodetector because producing fast photodetectors would enable additional applications,” said Fuentes-Hernandez – the aim is a hundred-fold improvement.
“Because we use materials that are processed from inks using printing techniques, they are not as ordered as crystalline materials,” said fellow researcher Professor Bernard Kippelen. “As a result, the carrier mobility and the velocity of the carriers that can move through these materials are lower, so you can’t get the same fast signals you get with silicon. But for many applications you don’t need picosecond or nanosecond response time.”
The active layer in the device sis 500nm thick – a gram could cover an office desk.
“Organic thin films absorb light more efficiently than silicon, so the overall thickness you need to absorb that light is very small,” Kippelen said. “Even if you scale their area up, the overall volume of your detector remains small with organics. If you increase the area of a silicon detector, you have a larger volume of materials that at room temperature will generate a lot of electronic noise.”