Prof Henry Snaith FRS

Abstract:

Voltage losses (ΔV_OC) are a crucial limitation for the performance of excitonic organic solar cells (OSCs) and can be estimated by two approaches─the radiative limit and the Marcus charge-transfer (MCT) model. In this work, we show that combining the radiative limit and MCT models for voltage loss calculations provides useful insights into the physics of emerging efficient OSCs. We studied nine different donor-acceptor systems, wherein the power conversion efficiency ranges from 4.4 to 14.1% and ΔV_OC varies from 0.55 to 0.95 V. For these state-of-the-art devices, we calculated the ΔV_OC using the radiative limit and the MCT model. Furthermore, we combined both models to derive new insights on the origin of radiative voltage losses (ΔV_rad) in OSCs. We quantified the contribution in ΔV_rad due to the bulk intramolecular (S₁) disorder and interfacial intermolecular (CT) disorder by revisiting the spectral regions of interest for OSCs. Our findings are in agreement with the expected relationship of V_OC with Urbach energy (E_U), which suggests that the low E_U is beneficial for reduced losses. However, unprecedentedly, we also identify a universal, almost linear relationship between the interfacial disorder (λ) and ΔV_rad. We believe that these results can be exploited by the organic photovoltaic (OPV) community for the design of new molecules and a combination of donor-acceptors to further improve OSCs.

Abstract:

Mixed halide perovskites can provide optimal bandgaps for tandem solar cells which are key to improved cost-efficiencies, but can still suffer from detrimental illumination-induced phase segregation. Here we employ optical-pump terahertz-probe spectroscopy to investigate the impact of halide segregation on the charge-carrier dynamics and transport properties of mixed halide perovskite films. We reveal that, surprisingly, halide segregation results in negligible impact to the THz charge-carrier mobilities, and that charge carriers within the I-rich phase are not strongly localised. We further demonstrate enhanced lattice anharmonicity in the segregated I-rich domains, which is likely to support ionic migration. These phonon anharmonicity effects also serve as evidence of a remarkably fast, picosecond charge funnelling into the narrow-bandgap I-rich domains. Our analysis demonstrates how minimal structural transformations during phase segregation have a dramatic effect on the charge-carrier dynamics as a result of charge funnelling. We suggest that because such enhanced recombination is radiative, performance losses may be mitigated by deployment of careful light management strategies in solar cells.