Film Flip and Transfer Process to Enhance Light Harvesting in Ultrathin Absorber Films on Specular Back‐Reflectors

Abstract

Optical interference is used to enhance light-matter interaction and harvest broadband light in ultrathin semiconductor absorber films on specular back-reflectors. However, the high-temperature processing in oxygen atmosphere required for oxide absorbers often degrades metallic back-reflectors and their specular reflectance. In order to overcome this problem, a newly developed film flip and transfer process is presented that enables high-temperature processing without degradation of the metallic back-reflector and without the need of passivation interlayers. The film flip and transfer process improves the performance of photoanodes for photoelectrochemical water splitting comprising ultrathin (<20 nm) hematite (α-Fe O ) films on silver-gold alloy (90 at% Ag-10 at% Au) back-reflectors. Specular back-reflectors are obtained with high reflectance below hematite films, which is necessary for maximizing the productive light absorption in the hematite film and minimizing nonproductive absorption in the back-reflector. Furthermore, the film flip and transfer process opens up a new route to attach thin film stacks onto a wide range of substrates including flexible or temperature sensitive materials.

Effect of Doping and Excitation Wavelength on Charge Carrier Dynamics in Hematite by Time‐Resolved Microwave and Terahertz Photoconductivity

“…Another study employing 4D electron energy loss spectroscopy suggested that a measured ≈3 ps signal was a signature of electron–hole recombination after a ligand field (LF or d–d) excitation . An ≈3 ps lifetime for all photogenerated carriers would, however, be inconsistent with reported quantum efficiencies of hematite photoanodes as well as reported collection lengths in photoelectrochemical measurements . A more probable cause for the ≈3 ps lifetime is carrier localization, which has been previously suggested to occur in either polaronic or mid‐gap states after LMCT excitation.…”

Semiconductor Thin Film Based Metasurfaces and Metamaterials for Photovoltaic and Photoelectrochemical Water Splitting Applications

“…Therefore, the specular reflectivity of the bottom mirror can be diminished during this annealing step. Recently, a film flip and transfer process was adopted to enable the deposition of bottom mirror after the annealing process . This process has been schematically depicted in Figure a.…”

“…a) The steps to fabricate Hematite based MOS cavity, b,c) the reflection spectra for MS cavities with different Hematite layer thicknesses, and d,e) the photocurrent values of the photoanode for different semiconductor layer thicknesses and doping conditions. Reproduced with permission . Copyright 2018, Wiley‐VCH.…”

Strong Interference in Planar, Multilayer Perfect Absorbers: Achieving High-Operational Performances in Visible and Near-Infrared Regimes

“…It is essential to develop a transfer technique that isolates the bottom layers from the annealing process. Recently, Kay et al developed a film flip and transfer process to make the coating of the metal layer after the annealing step [89], as shown in Figure 6(a). In addition to improving the electrical properties of the layer, the layer is homogeneously doped using Ti.…”