Back-contact barrier analysis to develop flexible and bifacial Cu(In,Ga)Se2 solar cells using transparent conductive In2O3: SnO2 thin films

The presence of a back-contact (BC) barrier disturbs the current density-voltage (J-V) characteristics of solar cells by limiting the majority carrier transport, which is commonly referred as rollover effect. In this study, the majority barrier height as high as 131 meV at the Cu(In,Ga)Se-2 (CIGS)/Mo-BC interface was calculated using the temperature dependent J-V characteristics. Subsequently, the lift-off process was applied to peel off the substrate type solar cells from Mo-BC to investigate the origin of the rollover effect. The MoSe2 layer was detected on liftoff Mo samples using energy dispersive X-ray spectroscopy and the grazing-incidence X-ray diffraction measurements. Finally, optically transparent In2O3:SnO2 (ITO) thin films were deposited on the lift-off CIGS samples to fabricate flexible and bifacial solar cells. The solar cell with ITO-BC showed improved fill factor and efficiency without rollover effect, owing to highly Ohmic-contact at CIGS/ITO interface. In this contribution, it has been demonstrated that MoSe2 layer, i.e. with unreferred crystallographic orientation, can be removed by lift-off process to develop bifacial CIGS solar cells on flexible substrates. The bifacial cell with ITO-BC showed efficiencies of 10.1% and 2.8% for frontside and backside illuminations, respectively.