Impact of the gate work function on the experimental I-V characteristics of MOS solar cells simulated with the Sentaurus TCAD software

Abstract

In this work, the influence of gate work function on the experimental J-VG characteristics of MOS solar cells was investigated with the aid of the Sentaurus TCAD for 2D numerical simulations of TiN/SiOxNy/Si Al/SiOxNy/Si and Al/MgO/Mg/SiOxNy/Si structures aiming at solar cells for energy harvesting applications. The increase of the gate work function led to the increase of the reverse current density as pointed out by the Sentaurus TCAD simulations and by the experimental J-VG characteristics. The work functions of Mg, Al, and un-annealed TiN used in the TCAD simulations were 3.7 eV, 4.1 eV, and 4.4 eV, respectively. It was observed that the onset voltage at 0.5 mA/cm2 in the forward-biasing region was at a lower voltage for TiN (~-0.06 V) compared to Al (~-0.42 V) and the Al/MgO/Mg stack (~-0.47 V). On the other hand, the current density increased steeply in the forward biasing for TiN and Al compared to the Al/ MgO/Mg stack gate and the thin MgO layer between Al and Mg worked as a potential barrier in an opposite direction to the potential barrier of the Mg/Si-OxNy/Si structure, which meant an onset voltage lowering for the Al/MgO/Mg/SiOxNy/Si solar cell. For the Al/MgO/Mg stack, the barrier effect of the MgO layer was fitted as a series resistance RS = 100 Ω and an equivalent Al/MgO/Mg work function of 4.15 eV considering a substrate doping NA = 1.2x1016 cm-3 and parallel conductance GP = 0. Also, the experimental JxVG characteristic of the Al/SiOxNy/Si cell was fitted for Al work function of 4.10 eV, a series resistance RS = 100 Ω, a parallel resistance RP = 0.02 Ω (GP = 50 S) and a substrate doping NA = 5.5x1015 cm-3. In this case, the high parallel conductance fitted was attributed to the tunneling through the dielectrics as a pre-dominant effect possibly caused by a high concentration of defects in the SiOxNy layer. Finally, the MOS solar cell parameters were relatively lower compared to those of commercial outdoor solar cells, but the power generated by the MOS cells reached the mW range, and the conversion efficiency from light energy into electrical energy was higher (12.7%) than the typical values found for energy-harvesting solar cells.