Comparison of Band Gaps and Open Circuit Voltages in Si/SiO2 Quantum Well Solar Cells

  • Robert Rölver, Institute of Semiconductor Electronics, RWTH Aachen University, Germany
  • Birger Berghoff, Institute of Semiconductor Electronics, RWTH Aachen University, Germany
  • Derk Bätzner, Institute of Semiconductor Electronics, RWTH Aachen University, Germany
  • Bernd Spangenberg, Institute of Semiconductor Electronics, RWTH Aachen University, Germany
  • Heinrich Kurz, Institute of Semiconductor Electronics, RWTH Aachen University, Germany

    • On the way to highly efficient new generations of Silicon based photovoltaics, new approaches in the solar cell material area are indispensable to overcome the limiting factors, i. e. hot carrier relaxation and IR photon transmission in todays bulk Si solar cells. One way to minimize hot carrier losses is the use of multiple absorbers with different band gap energies consisting of Si-quantum wells embedded between potential barriers .
    The approach shown here is the fabrication of Si/SiO2 multiple quantum wells (MQWs) by remote plasma enhanced chemical vapor deposition (RPECVD) and subsequent rapid thermal annealing (RTA) for recrystallization of Si layers. With this method it is possible to fabricate quantum wells with down to 1 nm thickness, where effective band gap energies are shifted to 1.6 eV as confirmed by photoluminescence (PL) and absorption measurements (Fig. 1, left).
    To overcome the inherent problem of poor conductivity in a MQW absorber containing insulating barriers an innovative solar cell design is explored. There, lateral transport parallel to the Si/SiO2 interfaces of 2 dimensional quantum wells is investigated.
    In a first attempt we fabricated lateral Si-QW solar cells with adjacent Al and Pt contacts resulting in an Schottky barrier induced internal field. Details of the increase in absorption energies shown in fig. 1 as afunction of QW thickness and of the corresponding open circuit voltages will be presented.