Bifacial Cu(In,Ga)Se2 solar cells using hydrogen-doped In2O3 films as a transparent back contact

Hydrogen-doped In2O3 (IOH) films are used as a transparent back contact in bifacial Cu(In,Ga)Se-2 (CIGS) solar cells. The effect of the IOH thickness and the impact of the sodium incorporation technique on the photovoltaic parameters are studied, and clear correlations are observed. It is shown that a loss in short circuit current density (J(SC)) is the major limitation at back side illumination. The introduction of a thin Al2O3 layer on top of the IOH significantly increases the collection efficiency (phi(x)) for electrons generated close to the back contact. In this way, the J(SC) loss can be mitigated to only similar to 25% as compared with front side illumination. The Al2O3 film potentially reduces the interface defect density or, alternatively, creates a field effect passivation. In addition, it prevents the excessive formation of Ga2O3 at the CIGS/IOH interface, which is found otherwise when a NaF layer is added before absorber deposition. Consequently, detrimental redistributions in Ga and In close to the back contact can be avoided. Finally, a bifacial CIGS solar cell with an efficiency (eta) of eta = 11.0% at front and eta = 6.0% at back side illumination could be processed. The large potential for further improvements is discussed.