arXiv subjects

Austin M. Kay

Publications and source records attributed to Austin M. Kay.

3 recordsLinked to original sources

arXiv2026

What Photocurrent Versus Effective Voltage Tells Us About Charge Generation in Organic Solar Cells

The photocurrent of an organic solar cell is routinely plotted against an effective voltage and used to extract an exciton dissociation probability, and through it a free-charge generation efficiency. The photocurrent measures neither. Using a drift-diffusion model in which free-charge generation is field independent and losses occur only by bimolecular recombination, we show that the apparent dissociation probability returned by this procedure lies well below unity even for complete dissociation. Across wide variations of carrier mobility and recombination strength it is a single-valued function of the fill factor and coincides with the short-circuit charge collection efficiency, reporting neither the generation yield nor anything about collection beyond what the fill factor already shows. Even for ideal transport the normalised photocurrent saturates below unity at an intensity-independent first-order limit set by recombination of photogenerated carriers with injected equilibrium charge. Applied to four organic solar cells whose generation efficiencies are known independently, the apparent dissociation probabilities track the short-circuit collection efficiency rather than the generation yield. We therefore suggest this construction not be used, particularly in the high-efficiency non-fullerene acceptor systems that are now state of the art.

physics.app-ph↗

arXiv2023

The Thermodynamic Limit of Indoor Photovoltaics Based on Energetically-Disordered Molecular Semiconductors

Due to their tailorable optical properties, organic semiconductors show considerable promise for use in indoor photovoltaics (IPVs), which present a sustainable route for powering ubiquitous "Internet-of-Things" devices in the coming decades. However, owing to their excitonic and energetically disordered nature, organic semiconductors generally display considerable sub-gap absorption and relatively large nonradiative losses in solar cells. To optimize organic semiconductor-based photovoltaics, it is therefore vital to understand how energetic disorder and non-radiative recombination limit the performance of these devices under indoor light sources. In this work, we explore how energetic disorder, sub-optical gap absorption, and non-radiative open-circuit voltage losses detrimentally affect the upper performance limits of organic semiconductor-based IPVs. Based on these considerations, we provide realistic upper estimates for the power conversion efficiency. The energetic disorder, inherently present in molecular semiconductors, is generally found to shift the optimal optical gap from 1.83 eV to ~1.9 eV for devices operating under LED spectra. Finally, we also describe a methodology (accompanied by a computational tool with a graphical user interface) for predicting IPV performance under arbitrary illumination conditions. Using this methodology, we estimate the indoor PCEs of several photovoltaic materials, including the state-of-the-art systems PM6:Y6 and PM6:BTP-eC9.

physics.app-ph↗

arXiv2021

Quantifying the Excitonic Static Disorder in Organic Semiconductors

Organic semiconductors are disordered molecular solids and as a result, their internal charge dynamics and ultimately, the performance of the optoelectronic devices they constitute, are governed by energetic disorder. To ascertain how energetic disorder impacts charge generation, exciton transport, charge transport, and the performance of organic semiconductor devices, an accurate approach is first required to measure this critical parameter. In this work, we show that the static disorder has no relation with the so-called Urbach energy in organic semiconductors. Instead, it can be obtained from photovoltaic external quantum efficiency spectra at wavelengths near the absorption onset. We then present a detailed methodology, alongside a computational framework, for quantifying the static energetic disorder associated with singlet excitons. Moreover, the role of optical interference in this analysis is considered to achieve a high-accuracy quantification. Finally, the excitonic static disorder was quantified in several technologically-relevant donor-acceptor blends, including high-efficiency PM6:Y6.

physics.app-ph↗