Fabrication of near-invisible solar cell with monolayer WS₂

Herein, we developed a near-invisible solar cell through a precise control of the contact barrier between an indium tin oxide (ITO) electrode and a monolayer tungsten disulfide (WS₂), grown by chemical vapor deposition (CVD). The contact barrier between WS₂ and ITO was controlled by coating various thin metals on top of ITO (M_x/ITO) and inserting a thin layer of WO₃ between M_x/ITO and the monolayer WS₂, which resulted in a drastic increase in the Schottky barrier height (up to 220 meV); this could increase the efficiency of the charge carrier separation in our Schottky-type solar cell. The power conversion efficiency (PCE) of the solar cell with the optimized electrode (WO₃/M_x/ITO) was more than 1000 times that of a device using a normal ITO electrode. Large-scale fabrication of the solar cell was also investigated, which revealed that a simple size expansion with large WS₂ crystals and parallel long electrodes could not improve the total power (P_T) obtained from the complete device even with an increase in the device area; this can be explained by the percolation theory. This problem was addressed by reducing the aspect ratio (width/channel length) of the unit device structure to a value lower than a critical threshold. By repeating the experiments on this optimized unit device with an appropriate number of series and parallel connections, P_T could be increased up to 420 pW from a 1-cm² solar cell with a very high value (79%) of average visible transmission (AVT).