In the past, as an n-type oxide semiconductor material, the company used zinc oxide (ZnO) or Ge-doped ZnO. However, the lowest potential of the bandgap conduction band is drastically lowered at the interface with Cu2O (p-type semiconductor), lowering output voltage by about 50%.
This time, by using an n-type oxide semiconductor material consisting of two materials and optimizing the compositions of the materials, Toshiba almost optimized the electric potential of the conductor. However, the company did not disclose the details of the two materials.
High efficiency, low cost might be realized at same time
Toshiba considers that it is possible to increase the conversion efficiency of the tandem-type PV cell to 30% or higher by further optimizing its materials and device structure. However, it is not possible to win in the market just by increasing efficiency. And the company aims to realize a PV cell with both a high efficiency and low cost (high cost effectiveness), it said.
There are some crystalline Si PV cells whose efficiency is higher than 23.8%, but their costs are very high. Some tandem-type PV cells using compound semiconductors such as GaAs and some multi-junction PV cells with more than two stories have an efficiency higher than 40%. However, their costs are several hundreds to several thousands times higher than those of crystalline Si PV cells.
On the other hand, Cu2O-based PV cells are expected to be manufactured by using very common materials at a low cost. By combining them with "mass-produced" crystalline Si PV cells having a high cost effectiveness, it might become possible to make a PV cell with an unprecedentedly high cost effectiveness.