# Quantum Dot Photovoltaic Power Generation

The solar cell technology is of extreme importance for global warming countermeasures. However, the energy conversion efficiency of solar cells is approaching the silicon-based theoretical limit of 29%, thus calling for the development of new materials or new structures to carry solar cells beyond this limit. Expectations are high that quantum dots will play an important role in breaking through the performance limit of existing solar cells. Ideally, the use of quantum dots should make it possible to raise the energy conversion efficiency to at least 60%. When light-collection systems are used, it is possible to generate the equivalent amount of electricity with 1/1,000 of an area of existing solar panels.

Semiconductor quantum dots are the innovative basic technology born in Japan in 1982 through research by Prof. Yasuhiko Arakawa and other researchers. In the solar cell mechanism, when sunlight hits semiconductors, electrons move to create the current of electricity. However, electrons move around freely on existing silicon-based semiconductors, and only electrons that reach electrodes can be taken out, creating the limit on electricity produced. Suppose that a box called "quantum dot," with a size of 10 nanometers (1 nanometer is one billionth of a meter), is placed in here. If the box's potential is low for electrons, electrons become confined in the box and lose the freedom of movement (See a chart on the right). Electrons confined in the box reach electrodes quite efficiently. We can control the energy of electrons (oscillation frequency) by changing the shape of the box. It is just like the way that wind instruments have different tones and pitches according to their shapes. As shown in a chart on the left side, we can change the properties of electrons freely by capturing and confining them in the small box called quantum dots.

Main factors for the energy loss that impose limits on the energy conversion efficiency of solar cells are the transmission loss from not being able to absorb energy from all wavelengths of solar light and the thermal loss that occurs when energy that is larger than the optical energy that can be accepted, causing energy to turn into heat within semiconductors. By utilizing the energy discreteness of quantum dots, it becomes possible to control the thermal loss. Through various forms of ingenuity, we can also eliminate the transmission loss. Through these steps, ideally, it is possible to achieve the energy conversion efficiency of 60% or over.

Up until now, compound semiconductors were mainly used as materials for quantum dots. But it is expected that silicon-based quantum dots will come into being ultimately. At the moment, however, there is a mountain of hurdles to clear before this can be achieved, and long-term research and development efforts required. For example, complete control over the size and location of quantum dots is essential, along with development of high-quality materials. Further, there remain a lot of things that need to be clarified fundamentally.

Quantum dots are an area of research where Japanese researchers have led the world in promoting research and development activities. Regarding the application of the technology to quantum dot solar cells, it is hoped that they can contribute to the realization of high-efficiency solar cells by further gathering up the excellence of intellectual capital in Japan. However, it would be dangerous to have bloated expectations in the short run and long-term research and development efforts over a span of 20 to 30 years are required. Going forward, Japan needs to establish the research and development structure for quantum dots, including the fostering of many researchers and developers from a broad spectrum of research areas. In the future, quantum dot solar cells will certainly be positioned as one of the most important infrastructure devices for the generation of green innovation.

## Conceptual diagram of quantum dot and electron microgram

Photo: Prof. Yasuhiko Arakawa, Director of the Institute for Nano Quantum Information Electronics (NanoQuine), University of Tokyo