Dr. Sam Teale

Abstract:

The epitaxial growth of a perovskite matrix on quantum dots (QDs) has enabled the emergence of efficient red light-emitting diodes (LEDs) because it unites efficient charge transport with strong surface passivation. However, the synthesis of wide-band gap (E_g) QD-in-matrix heterostructures has so far remained elusive in the case of sky-blue LEDs. Here, we developed CsPbBr₃ QD-in-perovskite matrix solids that enable high luminescent efficiency and spectral stability with an optical E_g of over 2.6 eV. We screened alloy candidates that modulate the perovskite E_g and allow heteroepitaxy, seeking to implement lattice-matched type-I band alignment. Specifically, we introduced a CsPb_1-xSr_xBr₃ matrix, in which alloying with Sr²⁺ increased the E_g of the perovskite and minimized lattice mismatch. We then developed an approach to passivation that would overcome the hygroscopic nature of Sr²⁺. We found that bis(4-fluorophenyl)phenylphosphine oxide strongly coordinates with Sr²⁺ and provides steric hindrance to block H₂O, a finding obtained by combining molecular dynamics simulations with experimental results. The resulting QD-in-matrix solids exhibit enhanced air- and photo-stability with efficient charge transport from the matrix to the QDs. LEDs made from this material exhibit an external quantum efficiency of 13.8% and a brightness exceeding 6000 cd m^-2.

Abstract:

Contemporary thin-film photovoltaic (PV) materials contain elements that are scarce (CIGS) or regulated (CdTe and lead-based perovskites), a fact that may limit the widespread impact of these emerging PV technologies. Tin halide perovskites utilize materials less stringently regulated than the lead (Pb) employed in mainstream perovskite solar cells; however, even today's best tin-halide perovskite thin films suffer from limited carrier diffusion length and poor film morphology. We devised a synthetic route to enable in situ reaction between metallic Sn and I₂ in dimethyl sulfoxide (DMSO), a reaction that generates a highly coordinated SnI₂·(DMSO)_x adduct that is well-dispersed in the precursor solution. The adduct directs out-of-plane crystal orientation and achieves a more homogeneous structure in polycrystalline perovskite thin films. This approach improves the electron diffusion length of tin-halide perovskite to 290 ± 20 nm compared to 210 ± 20 nm in reference films. We fabricate tin-halide perovskite solar cells with a power conversion efficiency of 14.6% as certified in an independent lab. This represents a ∼20% increase compared to the previous best-performing certified tin-halide perovskite solar cells. The cells outperform prior earth-abundant and heavy-metal-free inorganic-active-layer-based thin-film solar cells such as those based on amorphous silicon, Cu₂ZnSn(S/Se)₄ , and Sb₂(S/Se)₃.

Abstract:

2D/3D heterojunction perovskite solar cells have demonstrated superior efficiency and stability compared to their fully 3D counterparts. Previous studies have focused on producing 2D layers containing predominantly n = 1 perovskite quantum wells. In this report we demonstrate a technique to introduce dimensional mixing into the 2D layer, and we show that this leads to more efficient devices relative to controls. Simulations suggest that the improvements are due to a reduction in trap state density and superior band alignment between the 3D/2D perovskite and the hole-transporting layer.