Cu-based chalcopyrite semiconductors are promising for high-efficiency thin-film solar cells.

However, the band gap of highly efficient Cu(InGa)Se2 solar cells was about 1.15 eV, and an increase of the efficiency is expected by increasing the band gap energy.

But, the efficiency of solar cells with a band gap of about 1.4 eV is still lower than that of solar cells with a band gap of 1.15 eV.

The development of high-efficiency solar cells with a wide band gap absorber whose absorption coefficient matches to the solar spectrum is major challenging issue for researchers.

In the talk, I will discuss two new approaches to overcome the issue:

one is an improvement of quality of CIGS absorber by a novel growth method, and the other is an employment of a new buffer layer for CIGS solar cells.

In the new growth method of CIGS films, metal sources were intentionally activated during the evaporation. Since the activated species have higher kinetic energy and the release of the energy causes to the local heating of growing surface, we expected the migration enhancement of film precursors on the growing surface, resulting in the improvement of film quality. For the activation of metal species, we tried ionization of Ga atoms and Se cracking, and we obtained efficiencies of over 15% by using these active sources.

As a new buffer layer, we focus on Zn1-xMgxO (ZMO) alloy grown by the metal organic chemical vapor deposition (MOCVD).

We expected that ZMO film would be well adapted to chalcopyrite solar cells for improving their short-wavelength response, and open-circuit voltage.

The advantage of the MOCVD technique is a damage free process compared to the sputtering.

The effect of the growth temperature and Mg/(Mg+Zn) molar flow ratio on the crystalline structure of ZMO films was extensively investigated, and we have successfully grown ZMO films with controllable band gaps from 3.3 to 3.8 eV.

As a result, the solar cells with a ZMO buffer layer were fabricated without any surface treatment, and the efficiency of 12.5% was achieved.

Curriculum Vitae

Name: Akira YAMADA

Place and Date of Birth:

  Yamanashi Prefecture, Japan, April 3, 1961

Age: 47

Education:

  Bachelor of Engineering, Department of Physical Electronics,

  Tokyo Institute of Technology (1984)

  Master of Engineering, Department of Physical Electronics,

  Tokyo Institute of Technology (1986)

  Doctor of Engineering, Department of Physical Electronics,

  Tokyo Institute of Technology (1989)

Employment:

  Research Associate, Tokyo Institute of Technology (1989)

  Lecturer, Tokyo Institute of Technology (1990)

  Associate Professor, Tokyo Institute of Technology (1994)

  Visiting Researcher, Paul-Drude Institute

         (1995, Germany, 1 year)

Present:

  Professor

  Department of Physical Electronics,

         Tokyo Institute of Technology

Major research fields:

  1989- Low-Temperature Si Epitaxy by Photo-Chemical Vapor Deposition

  1991- Development of High-Efficiency Amorphous Silicon Solar Cells

  1991- Development of High-Efficiency Cu(InGa)Se2 Thin-Film Solar Cells

  2000- Development of MOSFET with a Strained-Si Channel

Awards:

  PVSEC Paper Award (PVSEC-12, 2001)

  WCPEC Poster Award (WCPEC-3, 2003)

  JJAP Editorial Contribution Award (JJAP, 2003)

  Best Paper Award (PVSEC-15, 2005)