Effect of an Ohmic back contact on the stability of Cu(In,Ga)Se2-based flexible bifacial solar cells

In this work, the influence of the Ohmic-contact behavior at the absorber/transparent back-contact interface on the stability of flexible bifacial Cu(In,Ga)Se-2 (CIGSe) solar cells is investigated. In the case of the CIGSe/In2O3:SnO2 (ITO) interface, the Ohmic contact is maintained via the introduction of trap-assisted recombination at the CIGSe surface during the ITO deposition. Post-annealed CIGSe/ITO interfaces are studied via time-resolved photoluminescence (TRPL). It is found that the TRPL lifetime of all the samples investigated drastically decreases after the ITO deposition owing to sputtering damage, whereas the TRPL lifetime tends to increase after heat treatment at 160 degrees C. This increase is attributed to the partial recovery of the sputtering damage during annealing; the increase is larger in samples with less severe sputtering damage than in samples with more severe sputtering damage. Flexible bifacial solar cells with Ohmic-like contact at the CIGSe/ITO interface show superior performance and long-term stability compared with those with non-Ohmic contact at the interface, which may be correlated with the alteration of the metastability during the ITO deposition and/or partial recovery of the sputtering damage. The best flexible bifacial CIGSe solar cells with Ohmic-like back contact show stable performance for over 70 days with efficiencies of 11.1% and 3.0% for the frontside and backside illuminations, respectively.