arXiv subjects

Uli Würfel

Publications and source records attributed to Uli Würfel.

3 recordsLinked to original sources

arXiv2026

Rethinking Charge Transport and Recombination in Donor-diluted Organic Solar Cells

We systematically investigate PM6:Y12 bulk-heterojunction solar cells with donor fractions ranging from 1% to 45%, linking morphology, charge transport, and recombination to device performance. Complementary structural and spectroscopic methods reveal that a percolating PM6 network forms even at below 5% donor content, with lamellar stacking and vertical composition gradients that do not hinder the charge extraction. The reduction of the effective active layer conductivity towards low donor fractions obeys a three-dimensional percolation model, indicating that charge transport is governed by network topology rather without a pronounced percolation threshold. A transition from nongeminate Langevin recombination to a dispersive Smoluchowski-type loss occurs below 5% donor fraction. The latter regime is also nongeminate, i.e., pertains to recombination of the total charge carrier density. Correspondingly, we observe that the Langevin reduction in the higher donor fractions - mostly dominated by redissociation of electron-hole pairs after encounter - changes towards low donor fractions: in these cases, the nongeminate loss rate exceeds the prediction of the Langevin model. This regime coincides with increasing transport resistance due to topology-limited hole conduction, leading to reduced fill factors despite a high retained charge-generation efficiency. Our results demonstrate that strong donor dilution preserves photogeneration if a continuous donor network is maintained, and unveil how topology-controlled transport and non-Langevin recombination jointly define the performance limits of donor-diluted organic solar blends.

cond-mat.mtrl-sci↗

arXiv2024

Transport resistance dominates the fill factor losses in record organic solar cells

Organic photovoltaics are a promising solar cell technology well-suited to mass production using roll-to-roll processes. The efficiency of lab-scale solar cells has exceeded 20% and considerable attention is currently being given to understanding and minimising the remaining loss mechanisms preventing higher efficiencies. While recent efficiency improvements are partly owed to reducing non-radiative recombination losses at open-circuit, the low fill factor due to a significant transport resistance is becoming the Achilles heel of organic photovoltaics. The term transport resistance refers to a voltage and light intensity dependent charge collection loss in low-mobility materials. In this Perspective, we demonstrate that even the highest efficiency organic solar cells reported to-date have significant performance losses that can be attributed to transport resistance and that lead to high fill factor losses. We provide a closer look at the transport resistance and the material properties influencing it. We describe how to experimentally characterise and quantify the transport resistance by providing easy to follow instructions. Furthermore, the causes and theory behind transport resistance are detailed. In particular, we integrate the relevant figures of merit and different viewpoints on the transport resistance. Finally, we outline strategies that can be followed to minimise these charge collection losses in future solar cells.

cond-mat.mtrl-sci↗

arXiv2021

Revealing Fundamentals of Charge Extraction in Photovoltaic Devices Through Potentiostatic Photoluminescence Imaging

The photocurrent density-voltage (J(V)) curve is the fundamental characteristic to assess opto-electronic devices, in particular solar cells. However, it only yields information on the performance integrated over the entire active device area. Here, a method to determine a spatially resolved photocurrent image by voltage-dependent photoluminescence microscopy is derived from basic principles. The opportunities and limitations of the approach are studied by the investigation of III-V and perovskite solar cells. This approach allows the real-time assessment of the microscopically resolved local J(V) curve, the steady-state Jsc, as well as transient effects. In addition, the measurement contains information on local charge extraction and interfacial recombination. This facilitates the identification of regions of non-ideal charge extraction in the solar cells and enables to link these to the processing conditions. The proposed technique highlights that, combined with potentiostatic measurements, luminescence microscopy turns out to be a powerful tool for the assessment of performance losses and the improvement of solar cells.

physics.app-ph↗