The interplay between exciton harvesting and charge photogeneration on the one side and charge recombination and transport on the other side determines the performance of organic solar cells. Nongeminate recombination is usually considered as reduced Langevin recombination. Langevin recombination describes the finding of electrons and holes in a disordered, homogeneous material by a rate proportional to the sum of electron and hole mobilities. The Langevin recombination rate is calculated based on experimentally determined charge carrier mobilities. The reduction factor is also calculated, as the fraction between the experimentally measured recombination rate and the Langevin recombination rate. Based on our earlier work [1], we try to shed light on what physical processes and properties of the solar cells actually lead to the reduction factor, including photogeneration and the actual interplay of mobilities for charge recombination [2] and, separately and differently, extraction [3]. We will then explain how nongeminate recombination, in combination with charge extraction, dominates the fill factor of organic solar cells.

    Charge transport in organic solar cells is inherently limited by the low conductivity typical for disordered semiconductors. As such, transport resistance represents a key loss mechanism, particularly for the fill factor (FF).^[1-3] While it is known that both electrons and holes contribute to this limitation, the appropriate formalism to describe their combined effect – by a harmonic or geometric mean – remains debated.
    We experimentally address this issue by studying PM6:Y12 blends with systematically varied donor content and temperature. Using single-carrier devices, we extract the individual electron and hole conductivities. From light-intensity-dependent current–voltage and open-circuit voltage measurements, we independently determine the effective conductivity under operating conditions using our recently proposed method.^[4]
    Our results show that the effective conductivity follows the harmonic mean of electron and hole conductivities across nearly three orders of magnitude in transport asymmetry. This trend holds across compositional and thermal variations, confirming that the slower carrier dominates charge extraction. The same harmonic relationship also applies to the effective mobility derived from space-charge limited current and resistance-dependent photovoltage measurements.
    These findings provide an experimentally grounded framework for analyzing charge transport in organic semiconductors, challenging the widespread use of geometric mean models. Understanding and correctly modeling transport losses is essential for identifying the rate-limiting species and optimizing material design. The harmonic mean emerges as the correct physical descriptor for transport-limited performance in low-mobility systems.

The aggregation behavior of dipyrrolonaphthyridinedione (DPND) chromophores in the solid state critically determines their optoelectronic properties. Here, we investigate how systematic variation in the side-chain geometry-specifically the branching point and steric profile-governs molecular packing and excitonic coupling. Using crystal structure prediction (CSP) combined with experimental GIWAXS and solid-state NMR, we obtain the packing geometry and crystal structure for six DPND derivatives, three of them are only the cores structures (DPND-iPr, DPND-EtPr₂, and DPND-iBu) and three other extended structures with a cyano-furan groups in order to increase the absorption range of these systems . The results for the cores molecules reveal that side-chain branching at the first carbon atom promotes herringbone packing and J-type behavior, while non-branching at the first carbon lead to an brick-wall stacking and H-type behavior in the solid state. For the extended molecules, it reveals that side-chain branching at the first carbon atom promotes a brick-wall stacking and J-type behavior, while non-branching at the first carbon lead to an herringbone packing and H-type behavior in the solid state, which indicate a strong structure-properties relationship between the branching in the first carbon and the aggregate behavior. Optical simulations based on the Holstein exciton-vibrational Hamiltonian[1] reproduce experimental absorption and photoluminescence spectra, confirming the transition from J-like to H-like photophysics as the side-chain branching position shifts. This study demonstrates that fine-tuning alkyl side-chain geometry enables rational control of aggregation and excitonic behavior in cross-conjugated DPNDs[2], providing new design principles for functional organic semiconductors.

The Power Conversion Efficiency (PCE) of Organic Photovoltaics (OPV) has recently crossed the 20% milestone, placing an even larger focus on module scale-up, as well as on device degradation and stability. In this presentation, recent work on scalable OPV will be presented, having a focus on Roll-to-Roll (R2R) techniques for scalable module development at ambient conditions [1,2]. We demonstrate ambient air slot-die coated OPV devices reaching above 15% PCE on cell and 13% PCE on module level, as well as a new device architecture facilitating >13% PCE for ITO-free devices manufactured using solely R2R processing techniques. Stability assessment is done using ISOS protocols to shed light on the degradation processes taking place in the OPV cells and modules. The results point at interface related degradation being dominant in these OPV devices, and degradation mechanisms taking place at device interlayers will here be discussed in more details. Finally, application routes in terms of transparent photovoltaic (TPV) and indoor OPV devices will be presented and discussed. This also includes activities on design and integration of photon light management stacks to enhance the performance of (semi-transparent) OPV modules further. 

Bulk heterojunction (BHJ) organic solar cells have achieved high efficiencies but suffer from poor morphological stability due to phase separation after long-term operation. Single-component OSCs based on double-cable polymers, offer improved stability through covalently linked donor and acceptor units. However, their efficiency remains limited by inefficient charge generation arising from extensive intermixed morphologies. Here, we report a fluorinated double-cable polymer, DCPY2-F, which achieves an outstanding efficiency of 14.8% with high short-circuit current density of 26.83 mA cm^-2. Ultrafast pump-probe transient absorption spectroscopy reveals that fluorination of DCPY2 into DCPY2-F accelerates interfacial charge transfer and long-range charge separation dynamics. The pump-push-probe spectroscopy and steady-state electroluminescence show that the faster interfacial charge transfer arises from a reduced reorganization energy and a correspondingly accelerated molecular reorganization process (2.5 ps vs. 0.8 ps). Despite comparable acceptor aggregate sizes with DCPY2, DCPY2-F also shows faster long-range charge separation dynamics, which we attribute to a narrower charge transfer states (CTs) energetic distribution. Molecular dynamics simulations further show that fluorination strengthens non-covalent interactions, promoting well-aligned intermolecular donor–acceptor interfaces. These structurally and energetically ordered interfacial CTs enable ultrafast, efficient charge generation, establishing interfacial energetic modulation via fluorination as a powerful strategy for high-efficiency and stable organic photovoltaics.

Organic photovoltaic molecules or polymers typically form semicrystalline thin films. It is widely recognized that the bulk heterojunction (BHJ) morphology of organic photovoltaic thin films, which consists of both crystalline and amorphous regions, plays a crucial role in determining the performance of devices. However, understanding the intricate three-dimensional multi-length scale morphology of these thin films remains a grand challenge. In this talk, we will present our recent progress on decoding the complex BHJ morphology of OPVs and developing strategies to control the morphology to enhance device performance. In addition to employing conventional techniques like GISAXS/GIWAXS, we will introduce two innovative methods: GTSAXS, which allows for the quantification of vertical nanomorphology, and GISANS combined with deuteration, a technique used to detect amorphous phase structures. Armed with these state-of-the-art scattering techniques, we investigated the optimal active layer morphology for OPVs, aiming at understanding the impacts of amorphous and crystalline phase structures on device performance and to advance the practical applications of these devices. Furthermore, these scattering techniques can also be applied in material science, chemistry, biology and condensed matter physics studies. By modifying the wavelength of the probing beam and the experimental geometry, a variety of sample types, such as solutions, powders, surfaces and thin films, can be studied, covering wide length scales as well as versatile dynamic and kinetic behaviors.

Here, we explore the morphology and exciton/charge carrier dynamics in bulk heterojunctions (BHJs) comprising various donor polymers and molecular acceptors. We analyze the influence of polymer-nonfullerene acceptor (NFA) blend composition on morphology, energetics, charge carrier recombination kinetics, and photocurrent properties. Transient absorption spectroscopy highlights the significance of an energetic cascade between mixed and pure phases in electron–hole dynamics to effectively separate spatially localized electron–hole pairs. Our findings suggest a correlation between the increase in NFA electron affinity in pure phases compared to mixed phases and a transition from a relatively planar backbone structure of NFA in pure, aggregated phases to a more twisted structure in molecularly mixed phases. Subsequently, we investigate charge carrier dynamics in organic photocatalytic nanoparticles designed for hydrogen evolution. Specifically, we explore the mechanism behind the dependence of photocatalytic activity on Pd content for a linear polymer. Our focus extends to the impact of Pd on the excited state of the polymer and the accumulation of long-lived charges during catalysis. Finally, we introduce a novel concept involving organic nanoparticles for visible light absorption and efficient charge separation.

Energy level alignment between organic semiconductors delicately depends on the interfacial morphology between donor and acceptor. Specifically, crystallization or aggregation, as well as amorphous blend phases, impact on the molecular energy levels and interfacial transitions through charge-transfer states. This has been already shown using a conventional polymer-fullerene bulk heterojunction. On the other hand, fluorination can modify both, the energy levels as well as the tendency of pristine phases for aggregation and crystallization. While fluorination may generally lead through increased electrostatic interaction to more tightly packed single phases, not all molecular sites are suitable to reach proper aggregation. Fluorination can thus also cause non-covalent conformational blocking instead of locking. We have investigated the impact of fluorination on single material energy levels – including the study of their optical and electronic properties. Comparison of the two allows us to detect exciton binding energies quantitatively – an otherwise hidden but important property of organic semiconductors. The influence of fluorination of polymeric donors or non-fullerene acceptors was investigated in more detail for model mixtures. Even though not directly detectable, energy level adjustments that occur in this process can be plausibly linked in particular to the influence of local electric fields due to quadrupolar molecular properties.

During the past decades, π-conjugated polymers (CPs) and oligomers have attracted extensive attentions on account of their peculiar optical properties, high conductivities and low-cost fabrication process. Poly(9,9-dioctylfluorene) (PFO) is a typical blue-emitting polymer capable of forming a β-phase conformation, which enhances conjugation length, chain packing, and the Förster radius for energy transfer. However, PFO suffers from photochemical instability and green-band emission under photo-oxidation. Here, Polydiarylfluorene-based conjugated polymers have emerged as versatile platforms for exploring the interplay between chain conformation, solid-state order, and photophysical gain processes. We present a unified investigation bridging structural evolution and optical amplification in these polymers. By combining calorimetry, grazing-incidence wide-angle X-ray scattering, and spectroscopic analyses, we reveal that thermally induced conformational ordering governs the emergence of emissive domains and the onset of amplified spontaneous emission. Complementary studies on phenyl-modified poly(diarylfluorene-co-N-phenyl) derivatives further demonstrate that controlled disruption of π–π stacking through chain twisting effectively suppresses exciton–exciton annihilation and enhances stimulated emission. These results highlight that both the establishment and modulation of conformational order are key to balancing exciton transport and gain stability in conjugated polymer films. Our findings provide fundamental insights into how molecular conformation dictates the photophysical pathways underlying light amplification in organic semiconductors, offering rational guidelines for the design of high-gain polymer emitters.

The power conversion efficiency of organic solar cells has recently surpassed 20%, yet a unified framework that captures the underlying photophysical mechanisms remains elusive. These devices operate through a complex excited-state choreography arising at the donor-acceptor interface where excitons, charge transfer (CT) and charge-separated states continuously interconvert according to the materials’ energetic landscape.

In this talk, we share our five-state kinetic model that, for the first time, explicitly incorporates the formation and re-splitting of local triplet excitons. Fully parameterized by the interfacial energy offset, this unified framework reproduces key photovoltaic observables – such as the charge generation efficiency, photoluminescence, electroluminescence and Langevin reduction factor. Our results indicate that the triplet state dynamics govern device performance across a wide range of energy offsets. In systems with moderate offset, triplet decay emerges as the dominant recombination pathway, reconciling long-standing experimental findings, including those in benchmark systems like PM6:Y6. The model further offers a mechanistic explanation for the empirically observed link between energy offset and non-Langevin recombination, and accurately predicts the device efficiency across different material systems. Notably, it identifies a singlet-CT offset of ~150 meV as optimal for efficient charge separation while suppressing loss pathways.

By connecting excited-state kinetics with macroscopic device metrics, our work offers a unified mechanistic picture of the photophysics in organic semiconductors, in particular the effects of energy offsets and disorder on the generation and recombination pathways of free charges. The insights gathered provide guidance for material and interface design strategies, aimed at overcoming the apparent efficiency ceiling in state-of-the-art organic solar cells.

Organic solar cells (OSCs) achieve 21% efficiency, yet non-radiative energy loss (qΔV_nr) remains a critical barrier to further improve the open-circuit voltage (V_OC). This loss is primarily governed by the optoelectronic properties of interfacial CT states, yet the precise role of electron-phonon coupling (EPC) is not fully resolved. Through analysis of three all-polymer OSCs and four small molecule acceptor (SMA)-based OSCs, we identify two donor/acceptor (D/A) interfacial mixed phases that foster two distinct CT states, establishing efficient charge generation. These two phases emerge from amorphous D/A entanglement, termed as Entangled (E-) interface, and the penetration of acceptor quasi-aggregates into donor polymer matrix, termed as Penetrated (P-) interfaces. The P-interface exhibits inherently weaker EPC than that of E-interface since the suppressed intramolecular interaction. As the results, the P-interfaces, governing all-polymer OSCs, achieve a significant reduction of ~60 meV in qΔV_nr compared to E-interface dominated SMA-based OSCs. The incorporation of PA into SMA system as guest component modulates the population of P-interface reducing the EPC and then enhancing V_OC. Overall, our work suggests that modulating the population of P-interfaces to suppress EPC is a viable strategy for reducing non-radiative voltage loss and overcoming the efficiency bottleneck of organic solar cells.