Toshiba Achieves 23.8% Efficiency With Tandem-type PV Cell

Toshiba Corp achieved a conversion efficiency of 23.8% with a tandem-type photovoltaic (PV) cell that uses cuprous oxide (Cu2O), etc for the top cell and crystalline silicon (Si) for the bottom cell.

Toshiba plans to increase the efficiency to 30% or higher in about 2022 for commercialization. The cell might become a new PV cell that realizes both a low cost and high efficiency.

This time, Toshiba developed a two-story PV cell called “tandem type.” With the two-story structure, it might become possible to convert sunlight into electricity more efficiently. The company achieved the conversion efficiency of 23.8% by using (1) a crystalline Si PV cell with a conversion efficiency of 22% for the first floor (bottom cell) and (2) a PV cell consisting of Cu2O (p-type semiconductor) and n-type oxide semiconductor for the second floor (top cell).

Toshiba announced almost the same tandem-type PV cell in January 2019, but its conversion efficiency was 22%. The efficiency was equivalent to that of the crystalline Si PV cell used for the bottom cell, and there was no advantage in making a tandem type. This time, the efficiency of the new PV cell is 1.8 points higher than the efficiency of the bottom cell, proving the value of the tandem type, the company said.

According to Toshiba, the reason why the conversion efficiency increased by 1.8 points in less than six months is that the company employed a new n-type oxide semiconductor material for the top cell.

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.

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.

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