Photovoltaic Device Team

Photovoltaic Power Team is developing technologies to fabricate highly efficient (>22%) and reliable solar cells. Specific issues to be addressed are as follows:

• Developing precise wafer slicing process
• Investigating new structures and processes to achieve high-efficiency solar cells including ion-implanting method
• Developing technologies to achieve higher efficiency and reliability of solar modules
• Establishing novel evaluation methods of solar cells and modules We are also engaging developments of solar cells with new concepts to realize over 30% efficiency. Specific structures include:
• Crystalline silicon-based multi-junction solar cells using "smart-stack" technology
• Heat-recovery solar cells, which we are theoretically and experimentally proposing

Research Outline

Further increase in efficiency and cost reduction of solar cells and modules are essential for the continuous spread and establishment of photovoltaic power generation. The photovoltaic power team maintains and operates all facilities to fabricate solar modules from crystalline silicon ingots. We are researching silicon wafers, solar cells, and solar modules. Specific research subjects are as follows:

A novel process for solar cell fabrication

We are developing a new cell fabrication process using ion implantation.
Our ion implantation technique using the stencil mask is expected to significantly reduce the number of production steps of solar cells while increasing their efficiency.

New concept solar cell: “Heat recovery solar cell”

Conventional solar cells discard more than half of solar energy as heat. We theoretically presented a new concept of nonequilibrium solar cell, heat recovery solar cell (HERC cell). HERC cell recovers thermal energy to allow its power conversion efficiency to exceed the Shockley-Queisser limit (29%) of silicon solar cells. We are conducting experiments to realize this new concept of solar cells.

Improvement in PV module reliability

We are investigating degradation modes of PV modules by accelerated aging tests to improve the reliability of PV modules. Our modules, using the double glass structure and the wiring with conductive films, showed improved reliability in dynamic mechanical load (DML) test.

Next-generation multi-junction solar cell “smart stack technology”

The “smart stack technology” using metal nanoparticle arrays has been developed, making the interconnection of various solar cells with different materials and bandgaps possible for the first time. This provides flexibility in material choice and device design because the mismatch in lattice constants, thermal expansion coefficients, etc. can be disregarded with this technique.

A GaAs/InP-based four-junction solar cell has achieved conversion efficiency as high as 31.6%, and a GaAs/CIGS-based three-junction solar cell has achieved conversion efficiency as high as 24.2% (joint research with the Research Center for Photovoltaics at AIST Tsukuba Center). We are working to improve and establish this technology for mass production.
The use of thin crystalline silicon as a bottom cell provides high efficiency and low-cost multi-junction cells. The team is developing crystalline silicon based smart stack cells that go beyond the theoretical efficiency limit of single-junction crystalline silicon solar cells (29%). A demonstration GaAs/Si three-junction cell with conversion efficiency of 25.1% has been successfully fabricated.

Main Research Facilities

【Fig. 6】Electrode firing furnace

Furnace for forming contacts between the electrode and the diffusion layer as well as Al‒BSF layers.

【Fig. 7】Spin etching apparatus

Apparatus that etches a single side of the wafer by spin rotation. Only one side can be etched without a protective film.

【Fig. 8】Ion implantation equipment

Equipment that implants accelerated phosphorus or boron ions in the wafer. The diffusion profile can be precisely controlled.

Major Achievements

1. Our bifacial back contact (BC) solar cell reached 22.1% efficiency. That is an efficient and production-friendly cell with screen-printed electrodes and thin wires.
2. We have established a self-aligning process for IBC solar cell fabrication using ion-implantation technology. Our IBC cells with their emitter, BSF, and FSF all fabricated by ion implantation reached 20.5% efficiency.
3. We have successfully fabricated passivated emitter and rear cells (PERC) with selective emitters using ion-implantation with stencil masks. Another type of our PERC cell with thermally diffused and etch-backed selective emitters showed very high open-circuit voltage.
4. We have indicated that eluted tin atoms from wiring solder accelerate the degradation of ethylene-vinyl acetate (EVA) sealant of the solar modules. Previously only acceleration of degradation by acetic acid was known.
5. We have applied the “smart stack” technology to GaAs/Si triple-junction solar cells and fabricated a cell with 27.7% efficiency.
6. We have proposed a theory of a new type of solar cell, heat recovery solar cell (HERC). We have proved that this kind of solar cell can achieve efficiency over limiting efficiency (29%) of silicon solar cell known as Shockley-Queisser limit.

【Fig. 9】Photograph and structure of the bifacial back contact (BC) silicon solar cell

Team Member