Challenges in advanced characterization techniques for high V_oc thin-film tandem solar cells

In this work we focus on two techniques developed to extract the individual open circuit voltages (V_oc) of tandem devices, as well as their sub-cell current–voltage (I–V) characteristics. These were developed to work on the micromorph cell, the subcells of which have only a limited region of overlapping photoresponse due to the very different band gaps of its constituent cells, which allows for selective illumination. However, it is not known if such techniques will work in tandem structures consisting of high V_oc structures (compared to the micromorph device), such as combinations of hydrogenated amorphous silicon oxide (a-SiO:H), polymorphous silicon (pm-Si:H), and amorphous silicon (a-Si:H) in a-SiO:H/pmSi:H a-Si:H/pm-Si:H device stacks. These novel structures have proven to suffer from less light induced degradation (LID), which is one of the main drawbacks of devices using a-Si:H. The presence of similar band gaps (difference of less than 150 mV in both cases) and overlapping external quantum efficiencies (EQEs) above 500 nm implies that some of the hypotheses used may not be valid for high V_oc devices. In this work we show the difficulties that arise when using these characterization techniques in such devices.