Cesium Lead Halide Perovskites with Improved Stability for Tandem Solar Cells

A semiconductor that can be processed on a large scale with a bandgap around 1.8 eV could enable the manufacture of highly efficient low cost double-junction solar cells on crystalline Si. Solution-processable organic–inorganic halide perovskites have recently generated considerable excitement as absorbers in single-junction solar cells, and though it is possible to tune the bandgap of (CH₃NH₃)­Pb­(Br_<i>x</i>I_1–<i>x</i>)₃ between 2.3 and 1.6 eV by controlling the halide concentration, optical instability due to photoinduced phase segregation limits the voltage that can be extracted from compositions with appropriate bandgaps for tandem applications. Moreover, these materials have been shown to suffer from thermal degradation at temperatures within the processing and operational window. By replacing the volatile methylammonium cation with cesium, it is possible to synthesize a mixed halide absorber material with improved optical and thermal stability, a stabilized photoconversion efficiency of 6.5%, and a bandgap of 1.9 eV

TiO₂ Conduction Band Modulation with In₂O₃ Recombination Barrier Layers in Solid-State Dye-Sensitized Solar Cells

Atomic layer deposition (ALD) was used to grow subnanometer indium oxide recombination barriers in a solid-state dye-sensitized solar cell (DSSC) based on the spiro-OMeTAD hole-transport material (HTM) and the WN1 donor-π-acceptor organic dye. While optimal device performance was achieved after 3–10 ALD cycles, 15 ALD cycles (∼2 Å of In₂O₃) was observed to be optimal for increasing open-circuit voltage (<i>V</i>_OC) with an average improvement of over 100 mV, including one device with an extremely high <i>V</i>_OC of 1.00 V. An unexpected phenomenon was observed after 15 ALD cycles: the increasing <i>V</i>_OC trend reversed, and after 30 ALD cycles <i>V</i>_OC dropped by over 100 mV relative to control devices without any In₂O₃. To explore possible causes of the nonmonotonic behavior resulting from In₂O₃ barrier layers, we conducted several device measurements, including transient photovoltage experiments and capacitance measurements, as well as density functional theory (DFT) studies. Our results suggest that the <i>V</i>_OC gains observed in the first 20 ALD cycles are due to both a surface dipole that pulls up the TiO₂ conduction band and recombination suppression. After 30 ALD cycles, however, both effects are reversed: the surface dipole of the In₂O₃ layer reverses direction, lowering the TiO₂ conduction band, and mid-bandgap states introduced by In₂O₃ accelerate recombination, leading to a reduced <i>V</i>_OC