Exciton formation dynamics and band-like free charge-carrier transport in 2D metal halide perovskite semiconductors
Advanced Functional Materials Wiley 33:32 (2023) 2300363
Abstract:
Metal halide perovskite (MHP) semiconductors have driven a revolution in optoelectronic technologies over the last decade, in particular for high-efficiency photovoltaic applications. Low-dimensional MHPs presenting electronic confinement have promising additional prospects in light emission and quantum technologies. However, the optimisation of such applications requires a comprehensive understanding of the nature of charge carriers and their transport mechanisms. This study employs a combination of ultrafast optical and terahertz spectroscopy to investigate phonon energies, charge-carrier mobilities, and exciton formation in 2D (PEA)2PbI4 and (BA)2PbI4 (where PEA is phenylethylammonium and BA is butylammonium). Temperature-dependent measurements of free charge-carrier mobilities reveal band transport in these strongly confined semiconductors, with surprisingly high in-plane mobilities. Enhanced charge-phonon coupling is shown to reduce charge-carrier mobilities in (BA)2PbI4 with respect to (PEA)2PbI4. Exciton and free charge-carrier dynamics are disentangled by simultaneous monitoring of transient absorption and THz photoconductivity. A sustained free charge-carrier population is observed, surpassing the Saha equation predictions even at low temperature. These findings provide new insights into the temperature-dependent interplay of exciton and free-carrier populations in 2D MHPs. Furthermore, such sustained free charge-carrier population and high mobilities demonstrate the potential of these semiconductors for applications such as solar cells, transistors, and electrically driven light sources.Optical Simulation-Aided Design and Engineering of Monolithic Perovskite/Silicon Tandem Solar Cells.
ACS applied energy materials 6:10 (2023) 5217-5229
Abstract:
Monolithic perovskite/c-Si tandem solar cells have attracted enormous research attention and have achieved efficiencies above 30%. This work describes the development of monolithic tandem solar cells based on silicon heterojunction (SHJ) bottom- and perovskite top-cells and highlights light management techniques assisted by optical simulation. We first engineered (i)a-Si:H passivating layers for (100)-oriented flat c-Si surfaces and combined them with various (n)a-Si:H, (n)nc-Si:H, and (n)nc-SiOx:H interfacial layers for SHJ bottom-cells. In a symmetrical configuration, a long minority carrier lifetime of 16.9 ms was achieved when combining (i)a-Si:H bilayers with (n)nc-Si:H (extracted at the minority carrier density of 1015 cm-3). The perovskite sub-cell uses a photostable mixed-halide composition and surface passivation strategies to minimize energetic losses at charge-transport interfaces. This allows tandem efficiencies above 23% (a maximum of 24.6%) to be achieved using all three types of (n)-layers. Observations from experimentally prepared devices and optical simulations indicate that both (n)nc-SiOx:H and (n)nc-Si:H are promising for use in high-efficiency tandem solar cells. This is possible due to minimized reflection at the interfaces between the perovskite and SHJ sub-cells by optimized interference effects, demonstrating the applicability of such light management techniques to various tandem structures.Rubidium Iodide Reduces Recombination Losses in Methylammonium‐Free Tin‐Lead Perovskite Solar Cells
Advanced Energy Materials Wiley 13:19 (2023)
Understanding the Degradation of Methylenediammonium and Its Role in Phase-Stabilizing Formamidinium Lead Triiodide.
Journal of the American Chemical Society American Chemical Society (ACS) 145:18 (2023) 10275-10284
Abstract:
Formamidinium lead triiodide (FAPbI<sub>3</sub>) 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 (MDACl<sub>2</sub>) has been used as an additive in FAPbI<sub>3</sub>. MDA<sup>2+</sup> has been reported as incorporated into the perovskite lattice alongside Cl<sup>-</sup>. However, the precise function and role of MDA<sup>2+</sup> remain uncertain. Here, we grow FAPbI<sub>3</sub> single crystals from a solution containing MDACl<sub>2</sub> (FAPbI<sub>3</sub>-M). We demonstrate that FAPbI<sub>3</sub>-M crystals are stable against transformation to the photoinactive δ-phase for more than one year under ambient conditions. Critically, we reveal that MDA<sup>2+</sup> is not the direct cause of the enhanced material stability. Instead, MDA<sup>2+</sup> degrades rapidly to produce ammonium and methaniminium, which subsequently oligomerizes to yield hexamethylenetetramine (HMTA). FAPbI<sub>3</sub> crystals grown from a solution containing HMTA (FAPbI<sub>3</sub>-H) replicate the enhanced α-phase stability of FAPbI<sub>3</sub>-M. However, we further determine that HMTA is unstable in the perovskite precursor solution, where reaction with FA<sup>+</sup> is possible, leading instead to the formation of tetrahydrotriazinium (THTZ-H<sup>+</sup>). By a combination of liquid- and solid-state NMR techniques, we show that THTZ-H<sup>+</sup> is selectively incorporated into the bulk of both FAPbI<sub>3</sub>-M and FAPbI<sub>3</sub>-H at ∼0.5 mol % and infer that this addition is responsible for the improved α-phase stability.Prospects for tin-containing halide perovskite photovoltaics
Precision Chemistry American Chemical Society 1:2 (2023) 69-82