Enhanced optical path and electron diffusion length enable high-efficiency perovskite tandems

Bin Chen,Se-Woong Baek,Eui-Hyuk Jung,Thomas G Allen,Makhsud I Saidaminov,Benjamin Scheffel,Erkan Aydin,Emmanuel Van Kerschaver,Mingyang Wei,F Pelayo García De Arquer,Sjoerd Hoogland,Andrew Proppe,Michele De Bastiani,Ya-Kun Wang,Edward H Sargent,Ziru Huang,Stefaan De Wolf,Yi Hou

doi:10.1038/s41467-020-15077-3

Abstract

Tandem solar cells involving metal-halide perovskite subcells offer routes to power conversion efficiencies (PCEs) that exceed the single-junction limit; however, reported PCE values for tandems have so far lain below their potential due to inefficient photon harvesting. 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. 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 perovskite top cell combined with solution-processed colloidal quantum dot:organic hybrid bottom cell leads to a PCE of 24%; while coupling the perovskite cell with a silicon bottom cell yields a PCE of 28.2%.

Highlights

Tandem solar cells involving metal-halide perovskite subcells offer routes to power conversion efficiencies (PCEs) that exceed the single-junction limit; reported power conversion efficiency (PCE) values for tandems have so far lain below their potential due to inefficient photon harvesting
Carrier extraction is still limited in thick films made using the method; but we show that the addition of a Lewis base decreases the trap density and enhances the electron diffusion length so that it exceeds 2 μm
To quantify the additional optical losses originating from using transparent conductive electrodes (TCEs) as back electrodes, we employed transfer-matrix methods (TMM) simulations in rearmirror and semi-transparent devices

Summary

Introduction

Tandem solar cells involving metal-halide perovskite subcells offer routes to power conversion efficiencies (PCEs) that exceed the single-junction limit; reported PCE values for tandems have so far lain below their potential due to inefficient photon harvesting. We increase the optical path length in perovskite films by preserving smooth morphology while increasing thickness using a method we term boosted solvent extraction Carrier collection in these films – as made – is limited by an insufficient electron diffusion length; 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%. Efficient harvesting of the photons above the bandgap of each subcell is needed to realize the potential of the tandem concept[16] To this end, both front and back electrodes of the top cell need to be highly transparent to allow near-infrared (NIR) photons to reach the bottom cell; and such a configuration reduces the internal reflection of photons close to the perovskite bandedge, making the achievement of high absorption of above-bandgap photons in the top cell challenging—for this must be accomplished in a single pass through the active layer. With these top and bottom cells, we achieve a PCE of 24% in perovskite/CQD tandems and 28.2% in perovskite/silicon tandems

Methods

Results

Conclusion