The photovoltaic, which uses Gallium arsenide (GaAs) semiconductors, was developed at the Gwangju Institute of Science and Technology and has a thickness of approximately 1 micron. This is achieved by using a transfer printing method that does not require an interlayer adhesive, allowing for the decrease in thickness.
The GaAs solar microcells are held temporarily to a film stamp using a layer of photoresist. The backside of the cells contains the bottom electrode, which is applied via an e-beam evaporator. The cells are brought into direct contact with the substrate electrode, which is deposited on a polyimide film. A pressure of 80 kPa is applied to the structure for 20 minutes at 170 °C, allowing for bonding between the electrodes as well as melting of the photoresist, which creates a protective layer against delamination. The film stamp can then be peeled away, and the photoresist removed with acetone, leaving behind the ultrathin solar cell.
This method, as well as the structure of the GaAs cells themselves, allows for the incredible thinness of these cells. In particular, the base layer of the solar cell, usually 2–4 microns in thickness, has been reduced to a thickness of 0.7 microns, and the bottom contact layer, also usually 2 microns in thickness, has been reduced to 0.1 microns. Despite this reduction in thickness, the 1.04 micron vertical-type cells perform at an efficiency of 15.2 percent, an improvement over the 14 percent efficiency of typical 4.24 micron cells.
Because of their reduced thickness, these cells experience much less strain when they are bent, and they can be bent around objects as small as 1 mm in thickness. This is a significant advantage in applications where flexible cells are needed. For example, ultrathin cells embedded in clothing would be able to move with the fabric as it is worn. The flexibility of the cells could allow them to fit into the small form factor of wearable technology such as smartwatches and glasses. The research team of Juho Kim et al. even mentions “skin” as a possible application, implying that these microcells could someday power electronics embedded into the human body.
The research is published under the title “Ultra-thin flexible GaAs photovoltaics in vertical forms printed on metal surfaces without interlay adhesives” in the journal Applied Physics Letters.