All-transparent photoelectric devices using metal oxides
Photoelectric devices, which include solar cells, LEDs, and photosensors, convert electric power to light, or vice versa. One everyday example is the cell phone. When you touch an icon on the phone's screen, an installed battery provides electric energy, which is changed to light, allowing you to see the display. Your touch gives the locational information of the icon to the processor, enabling the corresponding application. A transparent conductor (that is, an electrically conductive and light-transparent material) acts as a bridge that connects your finger with the icon. Unlike an opaque metal conductor, the transparent version ideally has no color. In the case of the cell phone, a thin transparent conductor is contained in a coating placed over the cell phone glass during manufacture.
We considered how photodetection of UV light using transparent materials allows the transmission of visible light from the sun's rays, while at the same time removing unwanted UV rays (as with a pair of UV-resistant sunglasses, for example). With this concept in mind, we applied devices made from wide energy bandgap materials that actively block UV light but also allow the transport of the visible portion of the spectrum. We developed a highly transparent nickel oxide/zinc oxide (NiO/ZnO) functional device for UV detection, which has around 90% transparency and strong protection against UV exposure. Moreover, the wide bandgap material combination actively absorbs UV, making the material a strong candidate for UV photodetection in flame detection or missile defense strategies.
We have produced a new class of all-metal oxide transparent photoelectric devices (T-PEDs) from the NiO/ZnO junction, as shown in Figure 1. We developed nickel oxide for the p-type transparent layer, and zinc oxide for the n-type layer. We formed this heterojunction p-NiO/n-ZnO using widely available large-scale sputtering.1 Our device has no opaque metal electrode, enabling full transparency (visible light permission >80%). Furthermore, our methodology does not contain any toxic chemical processing, and uses only Earth-abundant materials.2 Our optimized device, which has a 50nm-thick nickel layer, can efficiently absorb UV light (λ= 400nm) and provides the highest responsivity (3.85A W−1) and excellent detectivity (9.6×1013 Jones). Our investigations revealed fast UV photodetection (24.2ms)—shown in Figure 1(b)—which we attributed to defects and a very high absorption coefficient.
For enhanced photodetection performance, we used a nanowire-scaffold surface to develop a nanostructure. A pre-deposited nickel layer induced the spontaneous growth of nanowires. We also grew sputtered indium tin oxide nanowires, which act as a functional transparent conductor to provide a large light-reactive surface, as shown in Figure 2. This enabled a microsecond response for UV detection.
To demonstrate our system, we developed a transparent solar cell that actively uses short-wavelength UV light (which has a higher solar energy than visible light). Using the large energy bandgap nickel oxide, we achieved a very high open-circuit voltage of above 1.2V. Large-scale transparent solar cells are already available (see Figure 3), and a demonstration of a 4-inch sample is shown in Figure 3(b). Moreover, we are investigating the realization of flexible and transparent solar cells for lightweight and wearable applications: see Figure 3(c).
Elsewhere, other wide bandgap indium tin oxide materials have been used as a nanoscale lens to focus the incident light onto a designated spot inside silicon. These results demonstrate the potential for improved silicon solar cell efficiency of 16% or higher2–4 for alternative approaches to conventional high-efficiency silicon solar cells.5
In summary, metal oxide materials may enable a new era of transparent technologies, allowing the enhanced performance of photoelectric devices. Our next step will be to consider how we might realize more lightweight solar power applications using these systems.
Joondong Kim is a professor in the Department of Electrical Engineering and a principal researcher at the Photoelectric and Energy Device Application Laboratory. He has published more than 110 research papers and holds more than 80 patents both domestically and internationally. His research covers the synthesis of functional materials and the design of high-performing devices, including photoelectric sensors and photovoltaics.
Malkeshkumar Patel is a postdoctoral researcher in the Photoelectric and Energy Device Application Laboratory. He earned his PhD in photovoltaic science and engineering from the School of Solar Energy, Pandit Deendayal Petroleum University, for development and studies of copper zinc tin sulfide and tin(II) sulfide materials for solar energy applications. His research interests include nanostructured semiconductor materials for visible light-transparent photoelectric devices and their application for solar energy conversions.
Hong-Sik Kim is a senior researcher in the Photoelectric and Energy Device Application Laboratory, and is pursuing a PhD program at Sungkyunkwan University. He has worked in the LED and solar cell industry for five years. His research interests include nanostructured and flexible semiconductors for photoelectric device applications.