crystallization caused low-quality films and Sn 2+ oxidation induced p-doping. [4,5] These are accompanied by defects generated in perovskites, which always leads to quick reduction of the PCE and durability of tin-lead PSCs. [6] Tremendous efforts have been made to reduce the intrinsic defects in Sn-Pb PSCs by developing perovskite processing methods and customizing additives. [7][8][9][10] With advances in past years, the PCE of Sn-Pb PSCs has been elevated to more than 23%, which is still smaller but close to the Pb-based counterparts. [11,12] However, much larger challenges remain in the long-term stability of Sn-Pb PSCs.One concerning observation is that Sn-Pb perovskite photovoltaic devices still degrade much faster under light than Pb perovskites even when they are stored in inert condition or fully encapsulated to separate moisture and oxygen. [13] Recent studies show that they are not only caused
As the most commonly used hole transport material (HTM)
in tin–lead
(Sn–Pb) perovskite solar cells (PSCs), poly(3,4-ethylenedioxythiophene)
polystyrenesulfonate (PEDOT:PSS) limits the power conversion efficiency
(PCE) and stability of the PSCs due to its acidic characteristics.
Herein, an easily synthesized polymer HTM poly[(phenyl)imino[9-(2-ethylhexyl)carbazole]-2,7-diyl]
(CzAn) with a shallow highest occupied molecular orbital (HOMO) level
of −4.95 eV is used in a p-i-n structure, methylammonium-free,
Sn–Pb PSC to replace PEDOT:PSS. Upon optimization using doping
and surface engineering, high quality Sn–Pb PSCs could be successfully
fabricated, boosting the PCE to 22.6% (stabilized PCE of 21.3%) compared
with 21.2% for PEDOT:PSS. The perovskite films prepared on the modified
CzAn HTM possess improved crystallinity, reduced trap-state density,
and larger carrier mobility resulting in PSCs with greatly improved
stability.
The efficiency of all-perovskite tandem devices falls far below theoretical efficiency limits, mainly because a widening bandgap fails to increase open-circuit voltage. We report on a bifacial all-perovskite tandem structures with an equivalent efficiency of 29.3% under back-to-front irradiance ratio of 30. This increases energy yield and reduces the required bandgap of a wide-bandgap cell. Open-circuit voltage deficit is therefore minimized, although its performance under only front irradiance is not ideal. The bifacial device needs a sputtered rear transparent electrode, which could reduce photon path length and deteriorate stability of Pb-Sn perovskites. Embedding a light-scattering micrometer-sized particle layer into perovskite to trap light, effectively increases absorptance by 5 to 15% in the infrared region. Using a nonacidic hole transport layer markedly stabilizes the hole-extraction interface by avoiding proton-accelerated formation of iodine. These two strategies together increase efficiency of semitransparent Pb-Sn cells from 15.6 to 19.4%, enabling fabrication of efficient bifacial all-perovskite tandem devices.
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