Self-assembling monolayers in mixed tin-lead perovskite solar cells
Isabella Taupitz a, Sebastian Berwig a, Yeonghun Yun a, Maxim Simmonds a, Alexandra Miaskiewicz a, Philipp Tockhorn a, Steve Albrecht a
a Helmholtz-Zentrum Berlin für Materialien, Hahn-Meitner-Platz, 1, Berlin, Germany
Materials for Sustainable Development Conference (MATSUS)
Proceedings of MATSUS Spring 2024 Conference (MATSUS24)
#PerTanCell - Perovskite Tandem Solar Cells
Barcelona, Spain, 2024 March 4th - 8th
Organizers: Kai Brinkmann and Felix Lang
Poster, Isabella Taupitz, 549
Publication date: 18th December 2023

Tandem solar cells are able to exceed the maximum efficiency (detailed balance limit) of single junction
devices by far. In all-perovskite tandem solar cells, two perovskite absorbers with different bandgaps
(narrow-bandgap, NBG with Eg<1.3 eV and wide-bandgap, WBG with Eg>1.5 eV) are paired for optimal
utilization of the solar spectrum whilst profiting from the excellent optoelectronic properties of
perovskite films. Recently, all-perovskite tandem solar cells have reached 28% certified power-
conversion efficiency and are quickly approaching that of silicon-perovskite tandem solar cells [1]. Yet,
losses at the interface between tin/lead NBG subcell and hole transport layer (HTL) still limit the
performance of all-perovskite tandem solar cells. Very recently a few approaches have been published
demonstrating a successful replacement of the standard HTL PEDOT:PSS, which is known for its
detrimental influence on device performance, by self-assembling monolayer (SAM) materials [2–6].
Nevertheless, the details of charge-carrier dynamics and other film characteristics such as
crystallization of SAM-based mixed Sn/Pb perovskite films still lack understanding.
In this work, we study the influence of the pseudo-halide additive lead-thiocyanate (Pb(SCN)2) on the
performance of PEDOT:PSS- and SAM-based mixed Sn/Pb perovskite devices. Additive engineering
with thiocyanate compounds has been a major research focus for NBG perovskites and led the way to
the current record efficiencies in single-junction NBG and all-perovskite solar cells. However, our study
shows that a perovskite composition which has been developed for PEDOT:PSS fails solar cell behavior
upon replacing PEDOT:PSS with SAM (PCE > 16 % vs. PCE < 2 %). Results from JV measurements, as
well as transient surface photovoltage (trSPV) and transient photoluminescence (trPL) measurements
suggest that charge carrier extraction is blocked in SAM-based, Pb(SCN)2-containing devices.
Remarkably, morphology analysis and absolute PL measurements show significantly enlarged grain
size, smooth surface morphology and increased quasi Fermi-level splitting (QFLS) of Pb(SCN)2
containing films when fabricated on SAM, compared to films without Pb(SCN)2 (~0.75 eV vs. ~0.85 eV).
In contrast, for films fabricated on PEDOT:PSS there are only minor differences. These findings point
out, that a mixed Sn/Pb NBG perovskite composition which has been developed for high performance
PEDOT:PSS-based devices and all-perovskite tandems might not show the same device characteristics
when brought in contact with other HTMs instead. In conclusion, our study demonstrates that a careful
re-assessment of additive engineering plays a crucial role when replacing the standard hole-transport
material PEDOT:PSS in NBG sub-cells of all-perovskite tandem solar cells

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