Yoshitaka Okada, professor at the Research Center for Advanced Science and Technology, the University of Tokyo, achieved a cell conversion efficiency of 26.8% by concentrating light by 72 times with an intermediate band quantum dot photovoltaic (PV) cell.

The previous record with an intermediate band quantum dot PV cell was 21.2%, which was achieved by concentrating light by 1,000 times.

This time, the open-circuit voltage, short-circuit current density and fill factor were 2.05V, 1193.3mA/cm2 and 78.8%, respectively. They were measured with a cell measuring 5 x 5mm at UL LLC (Taiwan).

Okada applied a new structure to improve conversion efficiency and reduce heat generated by concentrating light. Different from multi-junction (MJ) compound semiconductor PV cells, intermediate band quantum dot PV cells realize a large amount of current. However, it increases the amount of heat generated. As a result, when light concentration rate is increased, output lowers.

Therefore, Okada formed an InGaP layer on the quantum dot layer so that part of the light coming to GaAs around quantum dots is absorbed by the InGaP layer. Because the quantum dot layer and InGaP layer are connected in series, it became possible to decrease the amount of current and increase voltage.