Improvement in carrier collection at the i/n interface of graded narrow-gap hydrogenated amorphous silicon germanium solar cells

We examined different buffer layers at the i/n interface of narrow-gap amorphous silicon germanium alloy (a-SiGe:H)-based thin-film solar cells. These buffers included a conventional hydrogenated amorphous silicon (a-Si:H), an inversely graded hydrogenated amorphous silicon germanium, and a crystalline seed buffer (CSB). The solar cell with the CSB shows the highest performance, of 10%. The better carrier extraction at the rear side of the device is attributed to the role of the CSB layer. The effect of CSB thickness from 50 nm to 100 nm on cell performance was examined. Cell efficiency increased with the buffer thickness up to 80 nm and decreased with buffer thickness of 100 nm. This decrease can be attributed to increased defect densities of the buffer due to less efficient passivation of amorphous phase at the crystalline column boundaries.