Optimization of emitter and interface of amorphous silicon/crystalline silicon heterojunction solar cells

The optimization of amorphous silicon/crystalline silicon (a-Si:H/c-Si) heterojunction solar cells was performed in terms of emitter material and heterointerface. Various conditions were explored and monitored by a residual gas analyzer. In addition, a thin microcrystalline (μc-Si) buffer layer was inserted into the heterojunction to improve the interface by bandgap grading. The conduction behavior is strongly influenced by the defect distribution and band offset at the hetero-interface. The recombination process dominates at low forward bias (V > 0.3 V), whereas multi-step tunneling capture emission (MTCE) occurs in the higher bias region (0.3 < V > 0.55 V) until the conduction becomes space charge limited (V < 0.55 V). The reduced band offset at the interface increases current levels by the enhanced diffusion/emission process. The solar cell with a 700 Å a-Si:H top layer and a 200 Å μc-Si buffer layer shows the maximum solar cell efficiency of around 10 %, without an antireflective coating, by improving both the carrier transport and the red response of the cell.