(In)GaAsN materials and solar cells for super-high-efficiency multijunction solar cells

We have been studying concentrator multi-junction solar cells under Japanese Innovative Photovoltaic R&D program since FY2008 and EU-Japan Collaborative Research on CPV since 2011. In order to realize 40% and 50% efficiency, new approaches for novel materials and structures are being studied. InGaAsN is one of appropriate materials for 4- or 5-junction solar cell configuration because this material can be lattice-matched to GaAs and Ge substrates. However, present InGaAsN single-junction solar cells have been inefficient because of low minority-carrier lifetime due to N-related recombination centers and low carrier mobility due to alloy scattering and non-homogeneity of N. Present (In)GaAsN single-junction solar cells have been inefficient because of low minority-carrier lifetime due to N-related recombination centers such as N-H-V_Ga, (N-N)_As and so on, and low carrier mobility due to alloy scattering and non-homogeneity of N. To solve above problems we have been developing the CBE technique. We have focused especially on the N incorporation in GaAsN thin films grown by CBE as a function of growth temperature and substrate orientation. We have also proposed a new flow-rate modulation CBE (FM-CBE) method in order to increase N incorporation and to reduce C and H incorporation in films. By adapting CBE technique to grow (In)GaAsN thin films, higher mobility and longer minority-carrier lifetime compared to those grown by the other growth methods have been achieved. According to these electrical properties, more than 15% efficiency is expected in CBE grown homo junction (In)GaAsN solar cell although only 7.2% efficiency (Jsc=15.1 mA/cm², Voc=0.662 V, FF=0.65) GaAsN single-junction cells have been obtained. To solve above problems, We have characterized deep levels in grown GaAsN films by DLTS and defect behaviors have been clarified.