Dr Junke Wang

Abstract:

Additive engineering is among the most commonly used strategies to enhance the performance and stability of perovskite solar cells. Prior research often focused on optimizing device performance by using additives in the perovskite precursor solution to influence the rate of crystallization and film formation, but a fundamental understanding of the effect of additives on the stoichiometry of the absorber remains elusive. In this study, we reveal how additives affect the ABX3 stoichiometry of the perovskite absorber and its photovoltaic properties. We find that the solar cell performance of a wide-bandgap (1.77 eV) Cs0.2FA0.8Pb-(I0.6Br0.4)3 perovskite decreases when processed with either of two common additives, lead thiocyanate and lead chloride, because the additive disturbs the stoichiometry. Interestingly, the addition of excess formamidinium iodide (FAI) to the precursor solution can restore the initial ABX3 stoichiometry and fully recover the device performance. The excess of FAI that is required depends on whether the halide or pseudohalide additive is incorporated into the crystal lattice. Finally, we alter the stoichiometry of an additive-free perovskite absorber by inducing either an excess or a deficiency of FAI or lead iodide in the precursor and show that slight deviations from the ideal stoichiometry rapidly degrade the device performance. This work provides fundamental insights into the importance of bulk stoichiometry in perovskite absorbers and can serve as a basis for future rational additive engineering.

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:

July 4, 2025

Abstract:

Metal-halide perovskite solar cells have achieved power conversion efficiencies comparable to those of silicon photovoltaic (PV) devices, approaching 27% for single-junction devices. The durability of the devices, however, lags far behind their performance. Their practical implementation implies the subjection of the material and devices to temperature cycles of varying intensity, driven by diurnal cycles or geographical characteristics. Thus, it is vital to develop devices that are resilient to temperature cycling. This Perspective analyses the behaviour of perovskite devices under temperature cycling. We discuss the crystallographic structural evolution of the perovskite layer, reactions and/or interactions among stacked layers, PV properties and photocatalysed thermal reactions. We highlight effective strategies for improving stability under temperature cycling, such as enhancing material crystallinity or relieving interlayer thermal stress using buffer layers. Additionally, we outline existing standards and protocols for temperature cycling testing and we propose a unified approach that could facilitate valuable cross-study comparisons among scientific and industrial research laboratories. Finally, we share our outlook on strategies to develop perovskite PV devices with exceptional real-world operating stability.