Abstract
Mixed tin–lead halide perovskites have recently emerged as highly promising materials for efficient single- and multi-junction photovoltaic devices. This Focus Review discusses the optoelectronic properties that underpin this performance, clearly differentiating between intrinsic and defect-mediated mechanisms. We show that from a fundamental perspective, increasing tin fraction may cause increases in attainable charge-carrier mobilities, decreases in exciton binding energies, and potentially a slowing of charge-carrier cooling, all beneficial for photovoltaic applications. We discuss the mechanisms leading to significant bandgap bowing along the tin–lead series, which enables attractive near-infrared bandgaps at intermediate tin content. However, tin-rich stoichiometries still suffer from tin oxidation and vacancy formation which often obscures the fundamentally achievable performance, causing high background hole densities, accelerating charge-carrier recombination, lowering charge-carrier mobilities, and blue-shifting absorption onsets through the Burstein–Moss effect. We evaluate impacts on photovoltaic device performance, and conclude with an outlook on remaining challenges and promising future directions in this area.
Highlights
Mixed tin−lead halide perovskites have recently emerged as highly promising materials for efficient single- and multi-junction photovoltaic devices
Highest reported power conversion efficiencies (PCEs) for single-junction devices have relied on the exceptional performance of lead-based perovskites, which offer strong absorption,[2] long charge-carrier lifetimes and diffusion lengths,[3−5] and high defect tolerance.[6−8] the lowest bandgaps attainable for lead halide perovskites are around 1.5 eV,[9] higher than the value of ∼1.3 eV required for maximum theoretical efficiencies of single-junction devices.[10,11]
From a fundamental perspective, mixed tin−lead halide perovskites have much to offer compared with their lead-only counterparts
Summary
One prominent reason for tin−lead iodide perovskites being attractive for photovoltaic applications is their bandgap tunability in the range of 1.2−1.6 eV, which encompasses values required for optimum single-cell efficiencies (∼1.3 eV)[10,11] as well as suitable low-bandgap candidates for bottom cells in all-perovskite tandem devices.[17,18] As Figure. Such shifts present a hurdle to the long-term stability of tin−lead perovskite solar cells, unless they can be reliably prevented from occurring (e.g., by additives such as SnF2, or impermeable encapsulation) over the projected lifetime of the device
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