Photoluminescence spectrum from heterojunction with intrinsic thin layer solar cells: An efficient tool for estimating wafer surface defects

In heterojunction with intrinsic thin layer “HIT” solar cells, care has to be taken to passivate the defects on the wafer surface before the deposition of the doped amorphous emitter and back surface field layers. Otherwise, these defects would lead to a significant loss in cell performance. It is therefore important to estimate the magnitude of these defects. In the passivation studies undertaken experimentally, both wafer surfaces were cleaned either by a dip in a 5% HF solution for 30 s or by various dry plasma processes, before depositing intrinsic a-Si:H layers. Detailed electrical–optical modeling of the measured PL signal from these wafers (in a solar cell configuration) not only allows us to quantify the surface defects in each case, relative to e.g., the sample showing the highest PL signal, but also to calculate the solar cell output parameters and link them to the intensity of the PL signal and the defect density on the wafer surface.Dry plasma cleaning has the advantage that it can be done in situ. Our preliminary plasma cleaning studies in conjunction with modeling indicate that it is possible in this way to achieve the levels of surface passivation comparable to standard HF dip cleaning.

► PL signals from N-c-Si wafers measured after various surface cleaning processes.
► Modeling of PL signals undertaken to estimate the relative surface defect density.
► Intensity of PL signal linked to both wafer surface defects and solar cell output.
► Emitter/c-Si defects in particular strongly degenerate PL and cell performance.