Esther Hung

Abstract:

Formamidinium lead triiodide (FAPbI₃) is the leading candidate for single-junction metal-halide perovskite photovoltaics, despite the metastability of this phase. To enhance its ambient-phase stability and produce world-record photovoltaic efficiencies, methylenediammonium dichloride (MDACl₂) has been used as an additive in FAPbI₃. MDA²⁺ has been reported as incorporated into the perovskite lattice alongside Cl^-. However, the precise function and role of MDA²⁺ remain uncertain. Here, we grow FAPbI₃ single crystals from a solution containing MDACl₂ (FAPbI₃-M). We demonstrate that FAPbI₃-M crystals are stable against transformation to the photoinactive δ-phase for more than one year under ambient conditions. Critically, we reveal that MDA²⁺ is not the direct cause of the enhanced material stability. Instead, MDA²⁺ degrades rapidly to produce ammonium and methaniminium, which subsequently oligomerizes to yield hexamethylenetetramine (HMTA). FAPbI₃ crystals grown from a solution containing HMTA (FAPbI₃-H) replicate the enhanced α-phase stability of FAPbI₃-M. However, we further determine that HMTA is unstable in the perovskite precursor solution, where reaction with FA⁺ is possible, leading instead to the formation of tetrahydrotriazinium (THTZ-H⁺). By a combination of liquid- and solid-state NMR techniques, we show that THTZ-H⁺ is selectively incorporated into the bulk of both FAPbI₃-M and FAPbI₃-H at ∼0.5 mol % and infer that this addition is responsible for the improved α-phase stability.

Abstract:

Perovskite solar cells (PSCs) offer an efficient, inexpensive alternative to current photovoltaic technologies, with the potential for manufacture via high-throughput coating methods. However, challenges for commercial-scale solution-processing of metal-halide perovskites include the use of harmful solvents, the expense of maintaining controlled atmospheric conditions, and the inherent instabilities of PSCs under operation. Here, we address these challenges by introducing a high volatility, low toxicity, biorenewable solvent system to fabricate a range of 2D perovskites, which we use as highly effective precursor phases for subsequent transformation to α-formamidinium lead triiodide (α-FAPbI3), fully processed under ambient conditions. PSCs utilising our α-FAPbI3 reproducibly show remarkable stability under illumination and elevated temperature (ISOS-L-2) and “damp heat” (ISOS-D-3) stressing, surpassing other state-of-the-art perovskite compositions. We determine that this enhancement is a consequence of the 2D precursor phase crystallisation route, which simultaneously avoids retention of residual low-volatility solvents (such as DMF and DMSO) and reduces the rate of degradation of FA+ in the material. Our findings highlight both the critical role of the initial crystallisation process in determining the operational stability of perovskite materials, and that neat FA+-based perovskites can be competitively stable despite the inherent metastability of the α-phase.

Abstract:

Improved understanding of heterojunction interfaces has enabled multijunction photovoltaic devices to achieve power conversion efficiencies that exceed the detailed-balance limit for single-junctions. For wide-bandgap perovskites, however, the pronounced energy loss across the heterojunctions of the active and charge transport layers impedes multijunction devices from reaching their full efficiency potential. Here we find that for polycrystalline perovskite films with mixed-halide compositions, the crystal termination—a factor influencing the reactivity and density of surface sites—plays a crucial role in interfacial passivation for wide-bandgap perovskites. We demonstrate that by templating the growth of polycrystalline perovskite films toward a preferred (100) facet, we can reduce the density of deep-level trap states and enhance the binding of modification ligands. This leads to a much-improved heterojunction interface, resulting in open-circuit voltages of 1.38 V for 1.77-eV single-junction perovskite solar cells. In addition, monolithic all-perovskite double-junction solar cells achieve open-circuit voltage values of up to 2.22 V, with maximum power point tracking efficiencies reaching 28.6% and 27.7% at 0.25 and 1.0 cm2 cell areas, respectively, along with improved operational and thermal stability at 85 °C. This work provides universally applicable insights into the crystalline facet-favourable surface modification of perovskite films, advancing their performance in optoelectronic applications.

Abstract:

Doping with small densities of foreign ions is an essential strategy for tuning the optoelectronic properties of semiconductors, but the effects of doping are not well-understood in halide perovskites. We investigate the effect of Bi3+ and Sb3+ doping in lead–tin perovskites. Films doped with small amounts of BiI3 and SbI3 show greatly increased non-radiative recombination at precursor doping concentrations as low as 1 ppm for Bi3+ and 1000 ppm for Sb3+. We rationalize such behaviour by density functional theory (DFT) simulations, showing that these metal ions can be incorporated in the perovskite crystal by introducing deep trap levels in the band gap. Having found that very small amounts of Bi3+ greatly reduce the optoelectronic quality of lead–tin perovskite films, we investigate the presence of Bi impurities in perovskite precursor chemicals and find quantities approaching 1 ppm in some. In response, we introduce a facile method for removing Bi3+ impurities and demonstrate removal of 100 ppm Bi from a perovskite ink. This work demonstrates how the incorporation of small concentrations of foreign metal ions can severely affect film quality, raising the importance of precursor chemical purity.