This study shows 28.2% lab-scale mechanically stacked perovskite/silicon tandem solar cells in collaboration with Sargent Group from the University of Toronto - published in Nature Communications
Here we increase the optical path length in perovskite films by preserving smooth morphology while increasing thickness using a method we term boosted solvent extraction.
Boosted solvent extraction
The perovskite front cells need to have a very high absorption of above-gap photons; and the highest possible transmittance below-gap. The latter requirement indicates that both the front and back electrodes of the top cell need to have exceptional transparency in the near-infrared (NIR) and short-wavelength infrared. The first criterion – of complete above-bandgap absorption – needs to be accomplished in a single pass through the top-cell active layer, i.e. the usual double-pass configuration in single-junction cells is not available to the front cell in tandems.
Typical perovskite cells can make do with a 300 nm active layer thickness when they benefit from the double pass. Our modeling showed that to achieve similar absorption in a single pass, a 700 nm thick active layer – one that is morphologically homogeneous and has an excellent charge carrier extraction length – would be required.
Inhomogeneous films arose when we simply increased the precursor concentration with the goal of thick perovskite films. We reasoned that a new strategy, which we term boosted solvent extraction (BSE), could potentially produce thick films at a precursor concentration compatible with smooth morphology.
Carrier collection in these films – as made – is limited by an insufficient electron diffusion length; however, we further find that adding a Lewis base reduces the trap density and enhances the electron-diffusion length to 2.3 µm, enabling a 19% PCE for 1.63 eV semi-transparent perovskite cells having an average near-infrared transmittance of 85%.
The improved near-infrared response of the SHJ bottom cells
Here, we improve the NIR response of the bottom cells by utilizing the H-doped In2O3 front contacts and decrease the number of front contact fingers.
One of the key enablers of high NIR transmittance is replacing commercial ITO with previously-developed highly-conductive Zr-doped In2O3 (IZrO) TCOs, whose parasitic free carrier absorption is suppressed, for a given free carrier density, by virtue of its enhanced carrier mobility
Figure. Reflection and EQE spectra of reference and NIR-optimized SHJ cell's of KPV-LAB. Short dashed lines indicate the band-edge cut-off of perovskite front cell. (Reference: Chen, Bin, et al. "Enhanced optical path and electron diffusion length enable high-efficiency perovskite tandems." Nature Communications 11.1 (2020): 1-9.)