Device Simulation of 25.9% Efficient ZnO x N y ${\rm ZnO}_x{\rm N}_y$ /Si Tandem Solar Cell

Abstract

The novel, high electron mobility material ${\mathrm{ZnO}}_x{N}_y$ has been investigated theoretically as an absorber in a two-terminal tandem solar cell. In addition to its high mobility, ${\mathrm{ZnO}}_x{N}_y$ can attain sufficiently low carrier concentration to enable $pn$-junctions, and has a tunable bandgap around the 1.7 eV range. It is therefore suitable for pairing with a Si-based bottom cell. In addition to the ${\mathrm{ZnO}}_x{N}_y$ layer, the tandem cell consists of a ${\mathrm{Cu}}_{2}O$ emitter and a Si heterojunction bottom cell. A buffer layer is introduced between the emitter and absorber in the top cell to mediate a large valence band offset that resulted in a poor fill factor, $FF$. A ${\mathrm{ZnO}}_x{N}_y$ buffer layer bandgap of 1.5 eV gave the highest power conversion efficiency (PCE). The objective is to estimate the optimal performance of ${\mathrm{ZnO}}_x{N}_y$ in a tandem solar cell. The dependence of current–voltage ($J$–$V$) characteristics on thickness, mobility and carrier concentration in the ${\mathrm{ZnO}}_x{N}_y$ layer is evaluated, and found to yield maximum performance with 0.35 $\mu m$, 250 ${\mathrm{cm}}^2$ Vs^–1 and ${10}^{16}$ ${\mathrm{cm}}^{-3}$, respectively. Using these conditions, the $J$–$V$ parameters of the device under AM1.5 illumination are short circuit current density, $J_{SC}=17.76$ mA ${\mathrm{cm}}^{-2}$, open circuit voltage, $V_{OC}=1.74$ V, $FF=83.8\%$ and $\mathrm{PCE}=25.9\%$. With this, it is reported on, to the best of the knowledge, the first device simulation based on ${\mathrm{ZnO}}_x{N}_y$.