A research team at Georgia Tech has demonstrated that large-area organic photodiodes, produced from solutions at low temperatures, can detect several hundred thousand photons every second. The process, the researchers said, is conceptually similar to the magnitude of light reaching a human’s eye from a single star. The low-noise and highly flexible organic devices allowed the researchers to substitute arbitrarily shaped, large-area photodiodes for the type of complex arrays that silicon photodiodes — which are more conventional — necessitate. Those arrays are expensive to scale up for large-area applications.
Except for response time, the researchers’ devices delivered performance comparable to that of rigid silicon diodes operating in the visible light spectrum. In coating their materials onto large-area substrates with arbitrary shapes, the researchers reported demonstrable evidence that organic photodiodes offer advantages over silicon photodiodes in applications that require response times in the range of tens of microseconds.
Dark Current Reduction
Where organic electronics rely on a foundation of carbon-base molecule- or polymer-fabricated materials, unlike conventional inorganic semiconductors such as silicon, the organic photodiodes use polyethylenimine — a polymer surface modifier that contains amine. In application, polyethylenimine produced air-stable, low-work-function electrodes in photovoltaic devices. The photovoltaics were developed in the laboratory of Bernard Kippelen, the Joseph M. Pettit Professor at Georgia Tech.
The use of polyethylenimine was additionally shown to produce photovoltaics with low levels of dark current, the electrical current that flows through devices in dark settings. That result meant the new materials could be used successfully in photodetectors to acquire faint visible light signals.
Canek Fuentes-Hernandez, principal research scientist in the School of Electrical and Computer Engineering at Georgia Tech, told Photonics Media that the team initially theorized using polythylenimine while investigating ways to develop low-work-function electrodes for organic solar cells. The discovery was reported in a 2012 paper in Science.
“In addition to strong reductions of a conductor’s work function, we found that use of polyethylenimine-modified electrodes yielded photodiodes with a large rectification and a small dark current density under reverse bias,” Fuentes-Hernandez said. “Using equivalent circuit models, we started to rationalize the origin of such unique characteristics in the context of solar cells.”
Rigid and flexible photodiodes. Courtesy of Georgia Tech.
That work is reported in a Journal of Materials Chemistry A paper from 2014. At the time, the researchers realized many of the reported and tested approximations aimed at deriving photodetector-specific metrics, including the presence and amount of electronic noise and noise-equivalent power, were not physically justified, Fuentes-Hernandez said.
“Beyond the notable performance achieved by our organic photodiodes, a critical component of our work is the reassessment of the methods of characterization and the physical origins of the electronic noise in organic photodiodes,” he said.
“This improved methodology allowed clarification of the critical role played by polyethylenimine and enabled development of guidelines for future device optimization.”
The work has spanned the better part of the past decade. It has involved reducing levels of dark current to the point that measurement equipment has had to be reengineered to detect an electronic noise that corresponded to the fluctuation of a single electron in one millionth of a second.
Scalable for Application
Uses for the new devices, the researchers said, include in the application used for the detection of ionizing radiation by scintillation. Here, a phosphor emits a light flash when it is struck by a high-energy particle. Lowering the level of detectable light would improve device sensitivity, enabling it to detect lower radiation levels.
Detecting vehicular radiation emissions, for one, requires use of a large detector area — which would be easier to make from organic photodiodes than silicon photodiode arrays.
Another practical advantage: noise minimization in the scale-up process.
“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.”
The active layer used to construct the organic photodiodes is only 500 nm thick. A gram of the material could coat the surface of an office desk, and is approximately the size of a human fingertip.
X-ray applications, where medical personnel require use of the smallest levels of radiation possible to minimize delivered dosages, would also benefit from sensitive and large-area organic photodiodes. Lidar, too, is a potential application Fuentes-Hernandez identified.
“One [application] that has caught our imagination for a long time is to develop low-cost wide-aperture eye-like cameras,” Fuentes-Hernandez told Photonics Media. “Easy-to-fabricate, nonplanar photodetector arrays can be used to simplify imaging systems. Think for instance in the retina of a human eye, where photodetectors are distributed on a curved surface. This curved photodetector surface allows the eye to use a single lens to create an image, across a large field of view, that does not suffer from field curvature aberrations.”
Currently, the researchers are working on improving photodetector response time; faster photodetection would enable a wider range of future and forthcoming applications, Fuentes-Hernandez said, and the team is aiming to increase its understanding of the physical mechanism that limits the photodiode response time.
“There is a real need to develop photodetector technologies that are more scalable, and one of the motivations of this work is to advance organic technology that we know is cost-effective for scaling,” he said.
“Because we use materials that are processed from inks using printing techniques, they are not as ordered as crystalline materials,” Kippelen said. “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 organic photodiodes do currently show electronic noise current values in the tens of femtoampere range. They also show noise-equivalent power values of a couple of hundreds of femtowatts.
“Advances like this will allow us to change the conventional wisdom that switching to organic materials that can lead to scalable devices would mean giving up performance,” Kippelen said.
“We can’t anticipate all the new applications that could be.”
This research was supported by the Department of the Navy, Office of Naval Research; the MURI Center for Advanced Organic Photovoltaics; the Air Force Office of Scientific Research; the Department of Energy/National Nuclear Security Administration; the Consortium for Nonproliferation Enabling Capabilities; the Consortium for Enabling Technologies and Innovation; Chilean National Commission for Scientific and Technological Research; the Doctoral Fellowship program “Becas Chile” from the Colombian Administrative Department of Science, Technology, and Innovation through the program Fulbright-Colciencias; the National Science Foundation through the Research Experiences for Undergraduates program; and the Brazil Scientific Mobility Program through an Academic Training Opportunities grant.
The research was published in Science (www.doi.10.1126/science.aba2624).