Optimal transparent conductive oxides (TCOs) are essential to reduce parasitic absorption losses in optoelectronic devices. Hydrogenated indium oxides (IO:H, ICO:H, IWO:H) lead the race, as they have electron mobilities > 100 cm2/Vs, low absorption in the visible and near-infrared parts of spectra and they can be easily deposited over a wide range of substrates. During deposition, the introduction of water hampers the material crystallization, hence as deposited films have amorphous microstructure. After annealing at 200°C, these materials experience a phase transition and coalesce in big crystalline domains, which are linked to the high electron mobility. Nonetheless, the presence of water during deposition is a drawback to upscaling humidity sensitive technologies to production.
In this work, we propose an alternative TCO sputtered without the intentional introduction of water during deposition: zirconium-doped indium-oxide (or IO:Zr). Using a combination of optoelectronic characterization, high-end electron microscopy techniques, electron recoil dispersive analysis and Rutherford backscattering, we fully characterize the material and explain the fundamental mechanisms limiting its electron transport. Even without the intentional introduction of water during deposition, these films have an amorphous microstructure and after annealing at 200 °C form crystallites with average size ~ 320 nm. With an electron mobility > 100 cm2/Vs and free carrier density as high as 2.5 × 1020 cm-3, 100 nm-thick films have a wider bandgap (between 3.5 eV and 3.9 eV) than the afore mentioned In-based TCOs. In addition, we found that the main limiting mechanism of electron transport in IO:Zr is ubiquitous phonon scattering.
Motivated by the high conductivity of the material, we reduced the thickness of the films from 100 nm to 15 nm to further reduce the optical absorptance. The thinnest films have an optical absorptance close to the glass substrate in which they were deposited- while still presenting high electron mobility (50 cm2/Vs), and high free carrier density. Interestingly all films, from 100 nm down to 15 nm-thick films, show the presence of large crystalline grains after annealing at 200 °C.
Finally, to demonstrate the applicability of the material, IO:Zr thin films with different thickness were applied as front electrode in silicon- and perovskite-based solar cells; showing in all cases an improvement in current density thanks to high transparency of IO:Zr as compared to ITO.