High work function thermally stable OMO-based transparent electrode for optoelectronic applications

Abstract

Ultrathin metal films (UTMFs) have emerged as a potential contender as a transparent electrode (TE) in various optoelectronic applications. Oxide-metal-oxide (OMO) based TEs incorporating UTMFs are the new state-of-the-art approach towards fabricating high-performance indium-free TEs. Here, we have demonstrated an OMO-based indium-free TE utilizing nickel oxide (NiO) and silver (Ag) as oxide and metal films respectively. Optical constants of NiO and Ag obtained from spectroscopic ellipsometry have been utilized in determining the optimum thickness of alternate layers in the OMO stack using a MATLAB code based on the transfer matrix method. To minimize the reflection losses, 40 nm of NiO as films was deposited on either side of the Ag film. A highly transparent NiO/Ag/NiO (NAN) electrode has been obtained by the physical vapor deposition technique. The fabricated TE shows high transmittance >82% in the visible region along with a low sheet resistance below 6 Ω/□. To analyze the thermal stability of NAN-TE, it has been treated at different elevated temperatures. It was observed that NAN-TE shows better thermal stability in comparison to commercially available ITO-TE. The incorporation of NiO as an overcoat layer results in enhancing the work function to 5.35 eV as measured by Kelvin Probe Force Microscopy (KPFM). The high average visible transmittance, high work function, low sheet resistance, and high-temperature tolerance make the developed TE compatible with optoelectronic devices where high-temperature processing is necessary. To verify the same a bifacial perovskite solar cell is fabricated incorporating the NAN-TE as top TE and a power conversion efficiency (PCE) of 3.51% and 2.56% is obtained by illuminating the fabricated device from FTO and NAN side.