• Energy Research

The NDT-stabilized devices exhibited suppressed trap – assisted recombination and improved operational stability. Thus, NDT might serve as a prospective stabilizing additive for the development of efficient and stable organic solar cells.

We propose a new approach for assessing the lifetimes of perovskite photovoltaics based on daily energy output which accounts for reversible diurnal changes.

Upconversion – the absorption of two or more photons resulting in radiative emission at a higher energy than the excitation – has the potential to enhance the efficiency of solar energy harvesting technologies, most notably photovoltaics. However, the required ultra-high light intensities and the narrow absorption bands of lanthanide ions limit efficient solar utilisation. In this paper, we report results from exciting upconverters with concentrated sunlight at flux densities up to 2300 suns, where the radiation is restricted to photon energies below the bandgap of silicon (corresponding to a wavelength λ = 1200 nm). Upconversion to λ = 980 nm is achieved by using hexagonal erbium-doped sodium yttrium fluoride (β-NaYF4: Er3+) in a fluoropolymer matrix. Upconversion has a nonlinear relation with irradiance, therefore at a high irradiance a threshold occurs where the process becomes linear. For β-NaYF4:25%Er3+, we find a two-photon threshold under concentrated sunlight at 320 suns. Notably, this threshold is lower than under corresponding laser excitation and can be related to all resonantly excited Er3+ ion levels and excited stated absorption. These results highlight a pathway that utilises a far broader portion of the solar spectrum for photovoltaics.

AbstractImproving the long-term stability of perovskite solar cells is critical to the deployment of this technology. Despite the great emphasis laid on stability-related investigations, publications lack consistency in experimental procedures and parameters reported. It is therefore challenging to reproduce and compare results and thereby develop a deep understanding of degradation mechanisms. Here, we report a consensus between researchers in the field on procedures for testing perovskite solar cell stability, which are based on the International Summit on Organic Photovoltaic Stability (ISOS) protocols. We propose additional procedures to account for properties specific to PSCs such as ion redistribution under electric fields, reversible degradation and to distinguish ambient-induced degradation from other stress factors. These protocols are not intended as a replacement of the existing qualification standards, but rather they aim to unify the stability assessment and to understand failure modes. Finally, we identify key procedural information which we suggest reporting in publications to improve reproducibility and enable large data set analysis.