Free-standing 2.7 μm thick ultrathin crystalline silicon solar cell with efficiency above 12.0%

Abstract

In this paper, a record-breaking efficiency of 12.3% is experimentally demonstrated for a flexible free-standing, 2.7-μm-thick ultrathin crystalline silicon (c-Si) solar cell, which is the highest ever-reported sub 3 μm c-Si solar cells. The first breakthrough of this study is achieving an exceptionally small exposure feature size of 350 nm using photolithography stepper without any additional stringent requirements on wafer flatness or depth of focus (DOF) of the projection system. This small exposure feature size enables the fabrication of a light-trapping scheme: a design of close-packed 700-nm-period touching inverted-pyramid nanostructures for enhanced light absorption. Furthermore, to reduce contact recombination, this work is also first to report the integration of nickel oxide (NiOx) as hole-selective contacts in conjunction with the nano-structure light trapping scheme on a free-standing ultrathin cell. The efficacy of these two mechanisms to enhance ultrathin c-Si cell efficiency is characterized in terms of the short circuit current density and open circuit voltage. The free-standing cell also shows good mechanical flexibility and compliance. In addition to demonstrating silicon-compatible fabrication, this work shows the potential of ultrathin c-Si technology to not only reduce material cost, but also maintain competitive device performance.