High-Mobility In₂O₃:H Electrodes for Four-Terminal Perovskite/CuInSe₂ Tandem Solar Cells

Four-terminal (4-T) tandem solar cells (e.g., perovskite/CuInSe₂ (CIS)) rely on three transparent conductive oxide electrodes with high mobility and low free carrier absorption in the near-infrared (NIR) region. In this work, a reproducible In₂O₃:H (IO:H) film deposition process is developed by independently controlling H₂ and O₂ gas flows during magnetron sputtering, yielding a high mobility value up to 129 cm² V^-1 s^-1 in highly crystallized IO:H films annealed at 230 °C. Optimization of H₂ and O₂ partial pressures further decreases the crystallization temperature to 130 °C. By using a highly crystallized IO:H film as the front electrode in NIR-transparent perovskite solar cell (PSC), a 17.3% steady-state power conversion efficiency and an 82% average transmittance between 820 and 1300 nm are achieved. In combination with an 18.1% CIS solar cell, a 24.6% perovskite/CIS tandem device in 4-T configuration is demonstrated. Optical analysis suggests that an amorphous IO:H film (without postannealing) and a partially crystallized IO:H film (postannealed at 150 °C), when used as a rear electrode in a NIR-transparent PSC and a front electrode in a CIS solar cell, respectively, can outperform the widely used indium-doped zinc oxide (IZO) electrodes, leading to a 1.38 mA/cm² short-circuit current (J_sc) gain in the bottom CIS cell of 4-T tandems.