Fabrication of near-invisible solar cell with monolayer WS.

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 (WS), grown by chemical vapor deposition (CVD). The contact barrier between WS and ITO was controlled by coating various thin metals on top of ITO (M/ITO) and inserting a thin layer of WO between M/ITO and the monolayer WS, 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 (WO/M/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 WS crystals and parallel long electrodes could not improve the total power (P) 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 could be increased up to 420 pW from a 1-cm solar cell with a very high value (79%) of average visible transmission (AVT).