Organic Photovoltaics
For organic solar cells, our interest focuses on non-equilibrium phenomena that distinguish these devices from their inorganic counterparts. In contrast to essentially all other (inorganic) photovoltaic systems, the thermalization of photocreated charges in organic semiconductors is a slow process. Much of our work focuses on unraveling how this affects device performance.
Electro-optical spectroscopy of organic solar cells
Organic solar cells are photovoltaic generators made predominantly from carbon-based materials, such as plastic substrates, conductive and semiconducting molecules or polymers. While organic solar cells can be described in some ways similar to their inorganic -- commonly Silicon-based -- relatives, they work on fundamentally different principles. Those include non-bandlike transport, excitonic absorption properties, or charge separation at the phase interfaces of donor:acceptor semiconductor blends, that are yet to be fully understood. We are utilizing a highly sensitive, electro-optical absorption spectroscopy method to directly map the density of states in the blended phases and at their interface in the steady state to study how energetic disorder, vibrational transitions, and the effective temperature of
generated charge carriers influence the solar cell performance and efficiency.
Contact: Jasper Weiler and Clemens Göhler
Noise Spectroscopy on Organic Semiconductors
Noise, although often perceived as an unwanted artefact in a measurement, contains essential information about the charge transport in materials. Noise spectroscopy has been used as a non-destructive tool to study about dynamics of charge carriers, defects and correlations in the system. This project aims to set up sensitive noise spectroscopy and, hence, measure theoretical thermal (Johnson-Nyquist) noise and shot noise in organic semiconductors. Often in literature, the shot noise deviates from the theoretical value. This deviation, quantified as the Fano factor, is an exciting quantity that can be used to understand charge transport.
Contact: Priya Viji
Non-Equilibrium Phenomena in Organic Solar Cells
The question whether charge transport in operational organic photovoltaic (OPV) occurs far-from-equilibrium or not is of significant practical and fundamental importance. The charge transport in OPV modelled using kinetic Monte Carlo has shown that charge carriers undergo slow thermalisation and, hence, are ‘hotter’ than their inorganic counterparts. Probing the effective temperature of photo-created charge carriers in OPV using noise spectroscopy is expected to offer a unique way forward to measure the ‘hotness’ experimentally.
Contact: Priya Viji
Influence of active layer morphology on the organic photovoltaic performance
Most state-of-the-art organic solar cells are based on the simple bulk heterojunction morphology, for which donor and acceptor materials are dissolved and processed in a common solvent. While this approach has enabled power conversion efficiencies above 18%, the open circuit voltage VOC of these organic photovoltaics still falls considerably below the theoretical maximum. A prominent yet incompletely understood loss channel decreasing VOC is the thermalization of photogenerated charge carriers in the density of states, that is broadened by energetic disorder. Symmetric morphologies like classic bulk heterojunctions do not provide a preferential direction to the thermalizing photogenerated charge carriers and the excess energy of the photogenerated charge carriers is lost in an undirected, diffusive motion. Carefully designed morphologies like funnels or compositional gradients in the donor:acceptor ratio on the other hand can mitigate this loss channel by rectifying the diffusive motion. Understanding the relationship between morphology and solar cell performance is a key step towards future organic photovoltaics with power conversion efficiencies beyond 20%.
Contact: Constantin Tormann
Recombination Order in Organic Solar Cells
Organic Solar Cells provide an interesting and low-cost alternative to conventional inorganic photovoltaics. Upon illumination, excitons are created that separate into free electrons and holes at the donor-acceptor-interface and can be extracted to deliver electricity. The opposing loss mechanism is the recombination of charge carriers, which happens in the bulk both prior to and after charge separation. To understand these processes, the recombination order is crucially important, which we determine via kinetic Monte Carlo Simulations under variation of different parameters such as morphology, contacts and delocalisation. Simulation results are compared to experimental data obtained via the Steady State Bias Assisted Charge Extraction (BACE) method on P3HT:PCBM and PM6:Y6 solar cells using different light sources.
Contact: Constantin Tormann
Low Temperature Characteristics Of Organic Solar Cells
The low temperature behavior of organic solar cells yields important information about the underlying processes like charge transfer and exciton formation. The open circuit voltage is an experimentally well accessible proxy for these mechanisms, as it is deeply linked to the energy level of the bound charge transfer states which decay to the free charge carriers constituting the device current. Measuring the open circuit voltage at varying temperatures allows us to extrapolate it down to 0K, where it should coincide with the charge transfer state energy as all losses due to thermalization are suppressed. Varying the light intensity as well allows us to further challenge our theoretical understanding of these devices.
Contact: Tobias Krebs
Impedance Measurement of Organic Solar Cells
This project investigates the electrical characteristics of organic solar cells (OSCs) through advanced impedance spectroscopy. Utilizing the high precision and flexibility of a lock-in amplifier, this research methodically examines the frequency response of OSCs under various conditions, such as applied AC/DC voltage on both the light source as well as the OSCs.
The study aims to enhance the understanding of charge dynamics within OSCs. These insights are critical for optimizing the design and fabrication processes of OSCs and aim to push forward the efficiency of organic photovoltaics, making them more viable in commercial applications.
Contact: Jan Philip Ott