The remarkable performance of hybrid perovskite photovoltaics is attributed to their long carrier lifetimes and high photoluminescence (PL) efficiencies. High-quality films are associated with slower PL decays, and it has been claimed that grain boundaries have a negligible impact on performance. We used confocal fluorescence microscopy correlated with scanning electron microscopy to spatially resolve the PL decay dynamics from films of nonstoichiometric organic-inorganic perovskites, CH3NH3PbI3(Cl). The PL intensities and lifetimes varied between different grains in the same film, even for films that exhibited long bulk lifetimes. The grain boundaries were dimmer and exhibited faster nonradiative decay. Energy-dispersive x-ray spectroscopy showed a positive correlation between chlorine concentration and regions of brighter PL, whereas PL imaging revealed that chemical treatment with pyridine could activate previously dark grains.
We study the effects
of a series of post-deposition ligand treatments
on the photoluminescence (PL) of polycrystalline methylammonium lead
triiodide perovskite thin films. We show that a variety of Lewis bases
can improve the bulk PL quantum efficiency (PLQE) and extend the average
PL lifetime, ⟨τ⟩, with large enhancements concentrated
at grain boundaries. Notably, we demonstrate thin-film PLQE as high
as 35 ± 1% and ⟨τ⟩ as long as 8.82 ±
0.03 μs at solar equivalent carrier densities using tri-n-octylphosphine oxide-treated films. Using glow discharge
optical emission spectroscopy and nuclear magnetic resonance spectroscopy,
we show that the ligands are incorporated primarily at the film surface
and are acting as electron donors. These results indicate it is possible
to obtain thin-film PL lifetime and PLQE values that are comparable
to those from single crystals by control over surface chemistry.
Metal
halide perovskite semiconductors offer rapid, low-cost deposition
of solar cell active layers with a wide range of band gaps, making
them ideal candidates for multijunction solar cells. Here, we combine
optical and electrical models using experimental inputs to evaluate
the feasible performances of all-perovskite double-junction (2PJ),
triple-junction (3PJ), and perovskite–perovskite–silicon
triple-junction (2PSJ) solar cells. Using parameters and design constraints
from the current state-of-the-art generation of perovskite solar cells,
we find that 2PJs can feasibly approach 32% power conversion efficiency,
3PJs can reach 33%, and 2PSJs can surpass 35%. We also outline pathways
to improve light harvesting and demonstrate that it is possible to
raise the performances to 34%, 37%, and 39% for the three architectures.
Additionally, we discuss important future directions of research.
Finally, we perform energy yield modeling to demonstrate that the
multijunction solar cells should not suffer from reduced operational
performances due to discrepancies between the AM1.5G and real-world
spectrum over the course of a year.
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