The short-wavelength (shorter than the bandgap) light of conventional unijunction PV cells is lost due to the thermal relaxation while their long-wavelength light is transmitted and cannot be effectively used. This problem causes the performance limit of unijunction PV cells called "Shockley-Queisser limit."
On the other hand, the InGaN photoactive layer of the new PV cell is 300nm-350nm in thickness. Different from Si, InGaN is a direct transition type and has a high optical absorptance.
"With a thickness of about 100nm, it absorbs half of the irradiated light," Emura said.
It means that InGaN absorbs most of the light when its thickness is 300nm. Also, in consideration of the life of the carrier, the distance to the electrodes is short. As a result, phonon scattering hardly occurs, preventing thermal relaxation. In other words, it becomes possible to eliminate one of the two major loss factors that determine the Shockley-Queisser limit, Emura said.
The transmission loss of long-wavelength electromagnetic waves such as infrared light will still remain. But when the bandgap is lowered to 0.92eV by controlling the composition of In in InGaN, the energy ratio of the unusable infrared light can be lowered to 10% of the entire sunlight, he said.
Even with the 10% loss and other losses caused by, for example, reflected light, the overall loss will be 20-30%. In other words, in ideal conditions, it is theoretically possible to realize a PV cell whose conversion efficiency is 70-80%.
However, the PV cell is still in the theoretical stage.
"We have yet to make an actual cell and evaluate it," Emura said.