arXiv subjects

Christopher Wöpke

Publications and source records attributed to Christopher Wöpke.

7 recordsLinked to original sources

arXiv2026

Mobility-lifetime relation links photodegradation in spin-coated and gravure-printed organic solar cells

The degradation mechanisms of organic solar cells (OSCs) have been studied primarily in spin-coated, laboratory-scale devices, whereas scalable processing modifies the device architecture, active-layer morphology, and underlying charge-transport and recombination properties. Whether these changes also alter how solar cells degrade remains unclear. Here, we compare spin-coated and fully roll-to-roll-compatible gravure-printed PM6:Y12 solar cells during $\sim$1000 h of continuous illumination. Despite distinct initial properties and degradation signatures, the loss of power-conversion efficiency systematically follows the mobility-lifetime product $μτ$. Remarkably, ageing of the printed devices increases the recombination lifetime while strongly reducing charge-carrier mobility, showing that a longer lifetime alone does not imply improved device performance. The mobility reduction is accompanied by decreased PM6 lamellar order, whereas the additional open-circuit voltage loss originates predominantly from increased non-radiative recombination. Dark recovery further reveals a metastable contribution specific to the printed architecture. These results identify the mobility-lifetime product as a unifying physical descriptor for photodegradation, linking ageing-induced microscopic changes to macroscopic performance loss across spin-coated and scalable printed OSCs.

cond-mat.mtrl-sci↗

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↗

arXiv2026

Bridging the lab-to-fab gap in non-fullerene organic solar cells via gravure printing

Organic solar cells have reached record efficiencies with non-fullerene acceptors, yet their translation to industrial printing remains a critical bottleneck. Here we report the highest efficiency achieved for a fully roll-to-roll-compatible gravure-printed non-fullerene organic solar cell. High-performance blends are typically optimised under laboratory coating conditions, while roll-to-roll manufacturing imposes fundamentally different constraints on ink stability, drying dynamics, and multilayer integration. Whether these constraints intrinsically limit device physics has remained unresolved. Here, we demonstrate a gravure-printed PM6:Y12 solar cell architecture using commercially available materials and establish a quantitative framework that disentangles optical, recombination, and transport losses in printed devices. We find that favourable bulk morphology and exciton harvesting can be preserved under gravure printing and non-halogenated solvents. The dominant efficiency penalties arise instead from optical interference within the printed layer stack and slow charge transport. Our results demonstrate that the performance gap between laboratory and printed solar cells is originating from device architecture rather than the intrinsic physics of modern non-fullerene systems, providing a mechanistic roadmap for roll-to-roll manufacturing of non-fullerene solar cells.

cond-mat.mtrl-sci↗

arXiv2022

Power-law density of states in organic solar cells revealed by the open-circuit voltage dependence of the ideality factor

The density of states (DOS) is fundamentally important for understanding physical processes in organic disordered semiconductors, yet hard to determine experimentally. We evaluated the DOS by considering recombination via tail states and using the temperature and open-circuit voltage ($V_\mathrm{oc}$) dependence of the ideality factor in organic solar cells. By performing Suns-$V_\mathrm{oc}$ measurements, we find that gaussian and exponential distributions describe the DOS only at a given quasi-Fermi level splitting. The DOS width increases linearly with the DOS depth, revealing the power-law DOS in these materials.

physics.app-ph↗

arXiv2022

Traps and transport resistance: the next frontier for stable state-of-the-art non-fullerene acceptor solar cells

Stability is one of the most important challenges facing organic solar cells (OSC) on their path to commercialization. In the high-performance material system PM6:Y6 studied here, investigate degradation mechanisms of inverted photovoltaic devices. We have identified two distinct degradation pathways: one requires presence of both illumination and oxygen and features a short-circuit current reduction, the other one is induced thermally and marked by severe losses of open-circuit voltage and fill factor. We focus our investigation on the thermally accelerated degradation. Our findings show that bulk material properties and interfaces remain remarkably stable, however, aging-induced defect state formation in the active layer remains the primary cause of thermal degradation. The increased trap density leads to higher non-radiative recombination, which limits open-circuit voltage and lowers charge carrier mobility in the photoactive layer. Furthermore, we find the trap-induced transport resistance to be the major reason for the drop in fill factor. Our results suggest that device lifetimes could be significantly increased by marginally suppressing trap formation, leading to a bright future for OSC.

cond-mat.mtrl-sci↗

arXiv2021

Charge Photogeneration in Non-Fullerene Organic Solar Cells: Influence of Excess Energy and Electrostatic Interactions

In organic solar cells, photogenerated singlet excitons form charge transfer (CT) complexes, which subsequently split into free charge carriers. Here, we consider the contributions of excess energy and molecular quadrupole moments to the charge separation process. We investigate charge photogeneration in two separate bulk heterojunction systems consisting of the polymer donor PTB7-Th and two non-fullerene acceptors, ITIC and h-ITIC. CT state dissociation in these donor-acceptor systems is monitored by charge density decay dynamics obtained from transient absorption experiments. We study the electric field dependence of charge carrier generation at different excitation energies by time delayed collection field (TDCF) and sensitive steady-state photocurrent measurements. Upon excitation below the optical gap free charge carrier generation becomes less field dependent with increasing photon energy, which challenges the view of charge photogeneration proceeding through energetically lowest CT states. We determine the average distance between electron-hole pairs at the donor-acceptor interface from empirical fits to the TDCF data. The delocalisation of CT states is larger in PTB7-Th:ITIC, the system with larger molecular quadrupole moment, indicating the sizeable effect of the electrostatic potential at the donor-acceptor interface on the dissociation of CT complexes.

cond-mat.mtrl-sci↗

arXiv2020

Open-Circuit Voltage Limitation by Surface Recombination in Perovskite Solar Cells

Fundamental electronic processes such as charge-carrier transport and recombination play a critical role in determining the efficiency of hybrid perovskite solar cells. The presence of mobile ions complicates the development of a clear understanding of these processes as the ions may introduce exceptional phenomena such as hysteresis or giant dielectric constants. As a result, the electronic landscape, including its interaction with mobile ions, is difficult to access both experimentally and analytically. To address this challenge, we applied a series of small perturbation techniques including impedance spectroscopy (IS), intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) to planar $\mathrm{MAPbI_3}$ perovskite solar cells. Our measurements indicate that both electronic as well as ionic responses can be observed in all three methods and assigned by literature comparison. The results reveal that the dominant charge-carrier loss mechanism is surface recombination by limitation of the quasi-Fermi level splitting. The interaction between mobile ions and the electronic charge carriers leads to a shift of the apparent diode ideality factor from 0.74 to 1.64 for increasing illumination intensity, despite the recombination mechanism remaining unchanged.

cond-mat.mtrl-sci↗