Mathias Nyman

Ion migration is a key phenomenon that influences the performance and stability of metal halide perovskite solar cells (PSCs). In this work, we systematically study how ion mobility evolves under thermal stress at 85 °C, using capacitance-frequency (C-f) spectroscopy measured in the dark. Aided by drift-diffusion simulations, we demonstrate that the measured C-f spectra cannot be reproduced using a single anion density with a unique ion mobility, particularly in the low-frequency domain where nonideal behavior emerges. A double-Gaussian distribution of anion mobilities provides a substantially better fit even for fresh devices and remains necessary after prolonged aging under thermal stress. This evolution suggests that heat stress not only influences the redistribution of mobile ions but also induces morphology-related changes, such as interface modifications and interlayer degradation, which collectively alter the electrochemical response of the device. Furthermore, the experimental hysteresis index (HI) for fresh and aged devices cannot be captured by assuming a single ion mobility, indicating that a distribution of ion mobilities is necessary to fully describe hysteresis behavior. These findings elucidate the complex ion dynamics under thermal stress and point toward the presence of distinct ionic populations contributing to device behavior, with implications for improving PSC stability.

Organic solar cells based on non-fullerene acceptors (NFAs) hold great potential for low-cost energy production; however, their large-scale manufacturability and long-term stability still pose challenges. In this work, we clarify how the doping concentration in PM6:Y6-based solar cells evolves as a function of time stored in the ambient. Our results show a rapid increase in the doping concentration, reaching 3 × 1016 cm−3 within the first few hours of ambient exposure. After 100 h of exposure to the ambient, the doping concentration saturates, reaching levels up to 1017 cm−3. A variation of the active layer thickness indicates higher doping concentrations in thinner samples. Previous work has shown that doping concentrations on the order of 1017 cm−3 can have a drastic effect on device performance—either positive or negative, depending on other device parameters. Our findings highlight the importance of properly characterizing unintentional doping in organic solar cell devices, not only in pristine devices, but also as a function of device aging.

Improving thermal stability is of great importance for the industrialization of polymer solar cells (PSC). In this paper, we systematically investigated the high-temperature thermal annealing effect on the device performance of the state-of-the-art polymer:non-fullerene (PM6:Y6) solar cells with an inverted structure. Results revealed that the overall performance decay (19% decrease) was mainly due to the fast open-circuit voltage (V_OC, 10% decrease) and fill factor (FF, 10% decrease) decays whereas short circuit current (J_SC) was relatively stable upon annealing at 150 °C (0.5% decrease). Pre-annealing on the ZnO/PM6:Y6 at 150 °C before the completion of cell fabrication resulted in a 1.7% performance decrease, while annealing on the ZnO/PM6:Y6/MoO₃ films led to a 10.5% performance decay, indicating that the degradation at the PM6:Y6/MoO₃ interface is the main reason for the overall performance decay. The increased ideality factor and reduced built-in potential confirmed by dark J − V curve analysis further confirmed the increased interfacial charge recombination after thermal annealing. The interaction of PM6:Y6 and MoO₃ was proved by UV-Vis absorption and XPS measurements. Such deep chemical doping of PM6:Y6 led to unfavorable band alignment at the interface, which led to increased surface charge recombination and reduced built-in potential of the cells after thermal annealing. Inserting a thin C₆₀ layer between the PM6:Y6 and MoO₃ significantly improved the cells’ thermal stability, and less than 2% decay was measured for the optimized cell with 3 nm C₆₀.

In thin‐film photovoltaics, such as organic and perovskite solar cells, charge extraction selectivity is crucial. In order to improve selectivity, charge transporting layers (doped and undoped) are frequently used; however, it is not well understood how a charge transporting layer should be designed in order to ensure efficient extraction of majority carriers while blocking minority carriers. This study clarifies how well charge transporting layers with varying majority carrier conductivities block minority carriers. The charge extraction by a linearly increasing voltage technique is used to determine the surface recombination velocity of minority carriers in model system devices with varying majority carrier conductivity in the transporting layer. The results show that transporting layers with high conductivity for majority carriers do not block minority carriers—at least not at operating voltages close to or above the built‐in voltage, due to direct bimolecular recombination across the transporting layer–absorber layer interface. Design principles are furthermore discussed and proposed to achieve selective charge extraction in thin‐film solar cells using charge transporting layers.

Vacuum‐thermal evaporation (VTE) is a highly relevant fabrication route for organic solar cells (OSCs), especially on an industrial scale as proven by the commercialization of organic light emitting diode‐based displays. While OSC performance is reported for a range of VTE‐deposited molecules, a comprehensive assessment of donor:acceptor blend properties with respect to their photovoltaic performance is scarce. Here, the organic thin films and solar cells of three select systems are fabricated and ellipsometry, external quantum efficiency with high dynamic range, as well as OTRACE are measured to quantify absorption, voltage losses, and charge carrier mobility. These parameters are key to explain OSC performance and will help to rationalize the performance of other material systems reported in literature as the authors’ methodology is applicable beyond VTE systems. Furthermore, it can help to judge the prospects of new molecules in general. The authors find large differences in the measured values and find that today's VTE OSCs can reach high extinction coefficients, but only moderate mobility and voltage loss compared to their solution‐processed counterparts. What needs to be improved for VTE OSCs is outlined to again catch up with their solution‐processed counterparts in terms of power conversion efficiency.

The main losses in perovskite solar cells (PSCs) are often related to their charge-transport layers (CTLs). While it has been shown how losses related to charge extraction and recombination at the CTL interfaces can be minimized individually, a comprehensive picture is still lacking. In this work, we describe how recombination at interfaces is related to the distribution of electric potential within the device based on drift-diffusion simulations. Our results show that interface recombination in PSCs can only be fully understood when the whole device is considered. We find that devices with poor conductivity in the CTLs or small built-in potential will suffer from fast recombination at interfaces and that the interface recombination can be avoided by improving these. To illustrate our results, we present a framework that provides an intuitive way to understand electrical potential and compare losses in PSCs. We show that the simulation data can be well understood within the framework for the whole range of parameters studied.

Suppressing charge recombination is key for organic solar cells to become commercial reality. However, there is still no conclusive picture of how recombination losses are influenced by the complex nanoscale morphology. Here, new insight is provided by revisiting the P3HT:PCBM blend, which is still one of the best performers regarding reduced recombination. By changing small details in the annealing procedure, two model morphologies are prepared that vary in phase separation, molecular order, and phase purity, as revealed by electron tomography and optical spectroscopy. Both systems behave very similarly with respect to charge generation and transport, but differ significantly in bimolecular recombination. Only the system containing P3HT aggregates of high crystalline quality and purity is found to achieve exceptionally low recombination rates. The high‐quality aggregates support charge delocalization, which assists the re‐dissociation of interfacial charge‐transfer states formed upon the encounter of free carriers. For devices with the optimized morphology, an exceptional long hole diffusion length is found, which allows them to work as Shockley‐type solar cells even in thick junctions of 300 nm. In contrast, the encounter rate and the size of the phase‐separated domains appear to be less important.

Organic bulk‐heterojunction solar cells based on the newly developed nonfullerene electron acceptors have the potential for very low‐cost energy production. However, to enable large‐scale production with common printing techniques, the active layer thicknesses need to be increased by up to an order of magnitude, which is currently not possible without significant loss in performance. Herein, the requirements for making nonfullerene acceptor (NFA)‐based solar cells with thick active layers and high efficiencies are clarified. The charge carrier mobility, unintentional doping concentrations, and bimolecular recombination prefactor in the model high‐efficiency system PM6 (Poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl‐3‐fluoro)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione)]):Y6 (2,2′‐((2Z,2′Z)‐((12,13‐bis(2‐ethylhexyl)‐3,9‐diundecyl‐12,13‐dihydro‐[1,2,5]thiadiazolo[3,4‐e]thieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2‐g]thieno[2′,3′:4,5]thieno[3,2‐b]indole‐2,10‐diyl)bis(methanylylidene))bis(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile) are determined. The results are implemented in a combined electro‐optical device model, which is used to determine the effect of varying these parameters on the efficiency. The results show that a mobility imbalance and doping can lead to improved performance at large thicknesses, partially contradicting previous studies performed on fullerene‐based systems. The findings highlight the importance of determining electron and hole mobilities selectively, as well as characterizing recombination and doping concentrations.

The field of organic solar cells has recently gained broad research interest due to the introduction of non-fullerene small molecule acceptors. The rapid improvement in solar cell efficiency put increased demand on moving towards scalable device architectures. An essential step towards this is enabling thicker active layers for which the hole- and electron mobility and their ratio become increasingly important. In this work, we demonstrate selective charge-carrier mobility determination using the charge extraction by a linearly increasing voltage (CELIV) method. By tuning the contact properties of the solar cell diodes, the hole- and electron mobility is determined separately using the recently developed MIM-CELIV technique. Balanced mobility is measured both in non-fullerene and in ternary blends with the recently published PBBF11 polymer. The mobility results are confirmed by using the well-established MIS- and photo-CELIV techniques.