Function Analysis of the Phosphine Gas Flow for n-Type Nanocrystalline Silicon Oxide Layer in Silicon Heterojunction Solar Cells

The energy conversion efficiency (η) of silicon heterojunction (SHJ) solar cells is limited by the current losses in the layer stack on the illuminated side. To reduce these losses, hydrogenated nanocrystalline silicon oxide (nc-SiOx:H) was implemented as a window layer in SHJ solar cells. However, the integration of nc-SiOx:H in devices without degradation of fill factor (FF) is still a challenge. To optimize the electron performance of devices, the optoelectronic properties and microstructure of nc-SiOx:H were characterized and analyzed systematically. It was found that the PH3 gas fraction (fPH3) plays a big role on the microstructure, oxygen content, and phosphorus (P) doping efficiency of the films. The highest conductivity, 2.84 × 10–1 S/cm, is obtained at a moderate fPH3 with an optical band gap of 2.26 eV. A ternary model was creatively used to show the variation in the composition of nc-SiOx:H as tuning fPH3. The growth of crystalline phase was accelerated by the P dopants when fPH3 is low, but further increasing fPH3 leads to excessive P inactive dopants, causing a phase transition from nanocrystalline silicon to amorphous silicon in nc-SiOx:H. In this work, the best solar cell with an nc-SiOx:H window layer achieves an FF of 81.4%, a short current density (Jsc) of 39.8 mA/cm2, an open-circuit voltage (Voc) of 731 mV, and an η of 23.7% at the moderate fPH3. A decrease in FF and Jsc is shown with higher fPH3, which is the consequence of the increased front contact resistivity and decreased optical band gap of nc-SiOx:H window layer.