Design analysis of ZnO/cSi heterojunction solar cell

This work deals with the design evaluation of n-ZnO/p-cSi and p-ZnO/n-cSi heterojunction solar cells using two dimensional numerical computer aided design tool (TCAD). Various physical parameters of the layer structure such as doping and thickness of the ZnO and cSi layers have been studied. The device performance is evaluated by implementing special models (i.e., surface recombinations, interface traps, thermionic field emission tunneling model for carrier transport at the heterojunction etc) at the semiconductor-semiconductor interfaces. A current density of 47 mA/cm², open circuit voltage of 0.526 V and efficiency of 19% was achieved for n-ZnO/p-cSi heterojunction for 40 - 80 nm thick i-ZnO layer. An electron affinity value range between 4.0 - 4.5 gives the best performance which fairly justifies the experimental values of the electron affinity of ZnO. Compared to reference crystalline Si device, external quantum efficiency of n-ZnO/p-cSi was increased (because of the increased absorption in ZnO) for smaller wavelengths. Contrary to n-ZnO/p-cSi structure, p-ZnO/n-cSi heterojunction shows poor photovoltaic response because photogenerated holes cannot surmount the large potential barrier (i.e., valence band offset) and hence these holes remain uncollected at the respective contact of the device.