Efficient Carrier Injection from Amorphous Silicon into Crystalline Silicon Determined from Photoluminescence

For devices with intrinsic amorphous silicon layer on a crystalline silicon substrate, the light absorbed in the amorphous silicon layer can be weakly electronically coupled into the crystalline silicon base. Such carrier injection has previously been reported using measurements on finished devices containing numerous layers. Here we use the spectral response of photoluminescence, a contactless approach that has been previously shown to provide a relative measurement of a device’s internal quantum efficiency, to investigate carrier injection on simpler structures. In solar cells with intrinsic amorphous silicon front layers and a crystalline silicon base, such as common heterojunction solar cells, the effect of the absorption in the front layer should be reflected in the internal quantum efficiency. A highly absorbing front layer is expected to cause a drop in the internal quantum efficiency at short wavelengths. However, if electron hole pairs that are generated in the front layer are subsequently injected into the base, the optical losses will be reduced, resulting in a partial recovery of the internal quantum efficiency at short wavelengths. Here we quantify the efficiency of carrier injection from the intrinsic amorphous silicon front layer to the base, by measuring the spectral response of photoluminescence on heterojunction test structures and comparing it with the internal quantum efficiency expected from the absorption losses in amorphous silicon.