Tandem solar cells stack multiple absorbing layers with different bandgaps to reduce the thermalization energy loss from above-bandgap photons. Thus, the tandem structure offers a pathway to power conversion efficiencies (PCEs) that ultimately reside beyond the single-junction limit.

Recently, metal halide perovskites have attracted considerable attention in the photovoltaic research community, and the wide range of control over bandgap makes them particularly suited to tandem applications. High-efficiency tandems have been built with perovskite top cells and bottom cells made using silicon, colloidal quantum dots, organics, and smaller-gap perovskites.

 The perovskite front cells need to have very high absorption of above-gap photons; and the highest possible transmittance below-gap. The latter requirement indicates that both 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.

 Only by developing a route to fast removal of excess solvents at reduced spin-coating speeds were we able to develop sufficiently thick films with homogenous morphologies. We overcame a short electron diffusion length in prior films by reducing the trap density through the addition of a Lewis base.

 We report, as a result, a 19% PCE for 1.63 eV semi-transparent perovskite cells having an average near-infrared transmittance of 85%. The combination of high efficiency and NIR transmittance enables a four-terminal perovskite/colloidal quantum dot tandem PCE of 24%, and a four-terminal perovskite/silicon tandem PCE of 28.2%.