Madrid, 7 th February 2008 Confidential 1 IBPOWER kick-off Meeting “Growth and Fabrication of Intermediate Band Solar Cells designed for concentration” Corrie Farmer and Colin Stanley University of Glasgow

Madrid, 7 th February 2008 Confidential 2 Standard (Al,Ga)As heterojunction solar cell GaAs reference/IBSC layer structure  QD density ~1-2x10 10 cm -2  Si  -doping to give one electron per dot  layers of QDs Donor impurities QD WL n + -substrate n + -Al 0.2 Ga 0.8 As n-GaAs p+p+ p + -GaAs n-metal contact Metal contact on p + -GaAs layer SiN x ARC, Al 0.85 Ga 0.15 As window layer IBSC - InAs QD layers incorporated into n-GaAs “base”

Madrid, 7 th February 2008 Confidential 3 Overview of device fabrication Cross-sectional schematic

Madrid, 7 th February 2008 Confidential 4 Solar cell characteristics (one sun)  Good efficiency of GaAs reference cell.  Lower efficiency for prototype IBSC.  Reduced V OC.  Photocurrent from IBSC due to absorption of sub- bandgap photons (2+2) with E IB -E VB <E hf <E g.  Photocurrent due to absorption of two sub- bandgap photons (2+3), one with E hf ~E CB -E IB IB CB VB

Madrid, 7 th February 2008 Confidential 5 Plans for next 6 months  Design for concentration  Minimize series resistance  Increase the open circuit voltage, V oc  MBE growth of IBSC structure  Maximize IB-CB separation (total In content per QD layer)  Maximize current produced by absorption of sub-bandgap photons (total number of QD layers)  Optimize spacer thickness between QD layers (coupled or uncoupled QDs??)  Fabrication of solar cells using;  ~5 μm thick electro-plated Au-grid for top electrode  Optimal grid geometry  Two-layer ARC  Heat sink  Characterisation by PL/EL/FTPS using FTIR spectrometer

Madrid, 7 th February 2008 Confidential 6 Further afield – strain balancing on InP V M Ustinov et al., Semiconductors Vol. 31 (10), p1080, 1997 In(Al,Ga)As InAs QD layer In(Al,Ga)As layer with In fraction reduced below lattice-matched value InAs QDs embedded in In 0.53 Ga 0.47 As on InP InP At 300 K; E g (InP) = 1.35 eV, E g (InAlAs) = 1.46 eV In theory, this should permit an “unlimited” number of QD layers to be stacked one on top of the other