Design, synthesis and RAFT polymerisation of a quinoline-based monomer for use in metal-binding composite microfibres

Metal-binding polymer fibres have attracted major attention for diverse applications in membranes for metal sequestration from waste waters, non-woven wound dressings, matrices for photocatalysis, and many more. This paper reports the design and synthesis of an 8-hydroxyquinoline-based zinc-binding styrenic monomer, QuiBoc. Its subsequent polymerisation by reversible addition–fragmentation chain transfer (RAFT) yielded well-defined polymers, PQuiBoc, of controllable molar masses (6 and 12 kg mol−1) with low dispersities (Đ, Mw/Mn < 1.3). Protected (PQuiBoc) and deprotected (PQuiOH) derivatives of the polymer exhibited a high zinc-binding capacity, as determined by semi-quantitative SEM/EDXA analyses, allowing the electrospinning of microfibres from a PQuiBoc/polystyrene (PS) blend without the need for removal of the protecting group. Simple “dip-coating” of the fibrous mats into ZnO suspensions showed that PQuiBoc/PS microfibres with only 20% PQuiBoc content had almost three-fold higher loadings of ZnO (29%) in comparison to neat PS microfibres (11%)

Minimizing geminate recombination losses in small-molecule-based organic solar cells

Small-molecule-based organic solar cells (OSCs) are a recurrent alternative to polymer-based OSCs. Due to the higher purity and definition of small molecules compared to polymers, the morphological requirements can be more relaxed. Here, we present a series of novel rhodanine-based small-molecule electron donors and blend them with the standard acceptor PC70BM. By performing a target analysis on femtosecond spectroscopy data, we quantify the rates of geminate charge recombination. We are able to reproduce these rates by applying the Marcus–Levich–Jortner equation, using results from quantum chemical calculations. This shows that in a series of differently substituted compounds, one can correctly predict trends in geminate recombination rates by relying only on quantities that are easy to measure (cyclic voltammetry, optical spectra) or that can be calculated by relatively inexpensive methods such as (TD)DFT. Our method should thus accelerate the search for high-performance small-molecule photovoltaic blends

Practical limits of multijunction solar cells

Multijunction solar cells offer a path to very high conversion efficiency, exceeding 60% in theory. Under ideal conditions, efficiency increases monotonically with the number of junctions. In this study, we explore technical and economic mechanisms acting on tandem solar cells. We find that these mechanisms produce limitations that are the more pronounced the greater the number of junction is and, hence, limit the ideal number of junctions, as well as the corresponding efficiencies. Spectral variations induce current losses in series‐connected tandem solar cells. For Denver, we find that these losses reduce achievable harvesting efficiencies to 51% for non‐concentrated light and that they restrict the ideal number of junctions to less than nine. Independently operated solar cells suffer from optical losses with similar consequences. Even high optical efficiencies of 99% restrict the ideal number of junctions to below 10 and reduce achievable efficiencies by more than 10%. Only architectures with a sequential cell illumination are more resilient to these losses. Restricting available materials reveals that a sufficiently low band gap for the bottom cell of 0.9 eV or below is expedient to realize high efficiencies. Economic considerations show that five junctions or less are economically ideal for most conceivable applications.In theory, multijunction solar can achieve conversion efficiencies of more than 80% with efficiency values increasing monotonically with the number of junctions. In practical operation, the ideal number of junctions is finite though, even for ideal solar cells. In this study, we explore the impact of spectrum variations, non‐ideal optical elements and economic considerations on achievable efficiencies and the ideal junction number. image Bayerisches Staatsministerium für Wirtschaft und Medien, Energie und Technologie http://dx.doi.org/10.13039/501100006463Bavarian State Governmen

Reducing Voltage Losses in Organic Photovoltaics Requires Interfacial Disorder Management

Thanks to the introduction of non‐fullerene acceptors, efficiencies of organic photovoltaics are now approaching 20%. Closing the gap with inorganic photovoltaics requires minimizing voltage losses without penalizing charge extraction, for which microstructure control is crucial. However, the complex interplay between microstructure and charge generation, recombination, and extraction has so far not been unraveled. Here, a systematic study linking device performance to distinct microstructural features via machine learning is presented. Building bi‐layer devices allows to separately study the influence of aggregation and disorder on the energies and lifetimes of bulk and interfacial states. Unambiguous assignments of specific structural motifs to the device photophysics are thus possible. It is found that the control of aggregation‐caused quenching is decisive for the exciton splitting efficiency and thus the carrier generation. Furthermore, the static disorder at the donor–acceptor interface controls the nonradiative recombination by shifting the excited state population from the bulk toward the interface. Finally, the amount of disorder in the bulk is found decisive for charge extraction. The finding that charge generation, recombination, and extraction are controlled by distinct structural features, is the key to optimizing these motifs independently, which will pave the way for organic photovoltaics toward the detailed balance limit.In this work, the aggregation‐related photophysical properties of bulk and interfacial states are disentangled through the construction of bilayer organic solar cells. Machine learning is used to enable the assignment of specific structural motifs to elementary device photophysics. The results highlight those different aspects of structural features control loss in voltage, current, and fill factor. imageDeutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Hellenic Foundation for Research and Innovation http://dx.doi.org/10.13039/501100013209China Scholarship Council http://dx.doi.org/10.13039/50110000454

Bypassing the Single Junction Limit with Advanced Photovoltaic Architectures

Multijunction devices and photon up‐ and down‐conversion are prominent concepts aimed at increasing photovoltaic efficiencies beyond the single junction limit. Integrating these concepts into advanced architectures may address long‐standing issues such as processing complexity, microstructure control, and resilience against spectral changes of the incoming radiation. However, so far, no models have been established to predict the performance of such integrated architectures. Here, a simulation environment based on Bayesian optimization is presented, that can predict and virtually optimize the electrical performance of multi‐junction architectures, both vertical and lateral, in combination with up‐ and down‐conversion materials. Microstructure effects on performance are explicitly considered using machine‐learned predictive models from high throughput experimentation on simpler architectures. Two architectures that would surpass the single junction limit of photovoltaic energy conversion at reasonable complexity are identified: a vertical “staggered half octave system,” where selective absorption allows the use of 6 different bandgaps, and the lateral “overlapping rainbow system” where selective irradiation allows the use of a narrowband energy acceptor with reduced voltage losses, according to the energy gap law. Both architectures would be highly resilient against spectral changes, in contrast with two terminal multi‐junction architectures which are limited by Kirchhoff's law.A Bayesian optimization‐based simulation framework is presented, able to perform virtual optimization of multi‐junction solar cell architectures, both vertical and lateral, in combination with up‐ and down‐conversion materials. Microstructure effects on performance are explicitly considered using machine‐learned predictive models from high throughput experimentation on simpler architectures. Vertical and lateral architectures are proposed to surpass the single junction limit of photovoltaic energy conversion. image Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Solar Technologies go Hybrid http://dx.doi.org/10.13039/100012027Medizinische Fakultät, Friedrich‐Alexander‐Universität Erlangen‐Nürnberg http://dx.doi.org/10.13039/501100009508Helmholtz Association http://dx.doi.org/10.13039/50110000931

Quantifying the efficiency of Förster-assisted optical gain in semiconducting polymer blends by excitation wavelength selective amplified spontaneous emission

We study the correlation between Förster resonance energy transfer (FRET) and optical gain properties in conjugated polymer blends based on regioregular poly(3-hexylthiophene) (P3HT) and poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT). First, FRET dynamics are investigated with femtosecond transient absorption spectroscopy observing a sub-picosecond energy transfer from F8BT to P3HT (550 fs) at medium doping levels (40% wt P3HT in F8BT). Amplified spontaneous emission (ASE) is then characterized in blends upon exciting predominantly the host and guest polymers, respectively. The corresponding density of absorbed photons at threshold conditions is compared upon host or guest photoexcitation as a method to quantitatively determine the FRET-assisted ASE efficiencies. We observe a reduction in ASE efficiency upon host photoexcitation of 20%, in the best case, respect to guest photoexcitation. Our results confirm that even in strongly coupled host:guest mixtures delayed exciton population by energy transfer is subtle to losses ascribed to exciton-exciton annihilation

Lévy defects in matrix-immobilized J aggregates : tracing intra-and intersegmental exciton relaxation

L.L. thanks the EC for financial support by the cofunded Amarout program and the Spanish Ministry for economy and competitiveness (plan nacional, Project MultiCrom (CTQ2014-58801)). G.C. acknowledges financial support by the European Research Council (ERC-2011-AdG No. 291198). D.G.L. and D.C. thank the UK EPSRC for funding via research grant EP/M025330/1 “Hybrid Polaritonics”One-dimensional J aggregates present narrow and intense absorption and emission spectra that are interesting for photonics applications. Matrix immobilization of the aggregates, as required for most device architectures, has recently been shown to induce a non-Gaussian (Lévy type) defect distribution with heavy tails, expected to influence exciton relaxation. Here we perform two-dimensional electronic spectroscopy (2DES) in one-dimensional J aggregates of the cyanine dye TDBC, immobilized in a gel matrix, and we quantitatively model 2DES maps by nonlinear optimization coupled to quantum mechanical calculations of the transient excitonic response. We find that immobilization causes strong non-Gaussian off-diagonal disorder, leading to a segmentation of the chains. Intersegmental exciton transfer is found to proceed on the picosecond time scale, causing a long-lasting excitation memory. These findings can be used to inform the design of optoelectronic devices based on J aggregates as they allow for control of exciton properties by disorder management.PostprintPostprintPeer reviewe

Precise Control of Process Parameters for >23% Efficiency Perovskite Solar Cells in Ambient Air Using an Automated Device Acceleration Platform

Achieving high-performance perovskite photovoltaics, especially in ambient air relies heavily on optimizing process parameters. However, traditional manual methods often struggle to effectively control the key variables. This inherent challenge requires a paradigm shift toward automated platforms capable of precise and reproducible experiments. Herein, we use a fully automated device acceleration platform (DAP) to optimize the process parameters for preparing full perovskite devices using a two-step method in ambient air. Eight process parameters that have the potential to significantly influence device performance are systematically optimized. Specifically, we delve into the impact of the dispense speed of organic ammonium halide, a parameter that is difficult to control manually, on both perovskite film and device performance. Through the targeted design of experiments, we reveal that the dispense speed significantly affects device performance primarily by adjusting the residual PbI2 content in the films. We find that moderate dispense speeds, e.g., 50 {\mu}l/s, contribute to top-performance devices. Conversely, too fast or too slow speeds result in devices with relatively poorer performance and lower reproducibility. The optimized parameter set enables us to establish a Standard Operation Procedure (SOP) for additive-free perovskite processing under ambient conditions, which yield devices with efficiencies surpassing 23%, satisfactory reproducibility, and state-of-the-art photo-thermal stability. This research underscores the importance of understanding the causality of process parameters in enhancing perovskite photovoltaic performance. Furthermore, our study highlights the pivotal role of automated platforms in discovering innovative workflows and accelerating the development of high-performing perovskite photovoltaic technologies

In Situ Probing the Crystallization Kinetics in Gas‐Quenching‐Assisted Coating of Perovskite Films

The pursuit of commercializing perovskite photovoltaics is driving the development of various scalable perovskite crystallization techniques. Among them, gas quenching is a promising crystallization approach for high‐throughput deposition of perovskite films. However, the perovskite films prepared by gas‐quenching assisted blade coating are susceptible to the formation of pinholes and frequently show inferior crystallinity if the interplay between film coating, film drying, and crystallization kinetics is not fully optimized. That arguably requires a thorough understanding of how single processing steps influence the crystallization kinetics of printed perovskite films. Here, in situ optical spectroscopies are integrated into a doctor‐blading setup that allows to real‐time monitor film formation during the gas‐quenching process. It is found that the essential role of gas quenching treatment is in achieving a smooth and compact perovskite film by controlling the nucleation rate. Moreover, with the assistance of phase‐field simulations, the role of excessive methylammonium iodide is revealed to increase grain size by accelerating the crystal growth rate. These results show a tailored control of crystal growth rate is critical to achieving optimal film quality, leading to fully printed solar cells with a champion power conversion efficiency of 19.50% and mini solar modules with 15.28% efficiency are achieved.Utilizing in situ monitoring techniques to optimize the crystallization kinetics of the perovskite films in the gas‐quenching‐assisted blade coating process, a champion power conversion efficiency of 19.50% for a fully printed carbon‐electrode perovskite solar cell is achieved through the tailored control of crystal growth rates. image Bavarian State GovernmentNational Natural Science Foundation of China http://dx.doi.org/10.13039/501100001809Guangzhou Basic and Applied Basic Research FoundationBavarian Ministry of Science and ArtsDeutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/50110000165

On the critical competition between singlet exciton decay and free charge generation in non-fullerene based organic solar cells with low energetic offsets†

Reducing voltage losses while maintaining high photocurrents is the holy grail of current research on non-fullerene acceptor (NFA) based organic solar cell. Recent focus lies in understanding the various fundamental mechanisms in organic blends with minimal energy offsets – particularly the relationship between ionization energy offset (ΔIE) and free charge generation. Here, we quantitatively probe this relationship in multiple NFA-based blends by mixing Y-series NFAs with PM6 of different molecular weights, covering a broad power conversion efficiency (PCE) range: from 15% down to 1%. Spectroelectrochemistry reveals that a ΔIE of more than 0.3 eV is necessary for efficient photocurrent generation. Bias-dependent time-delayed collection experiments reveal a very pronounced field-dependence of free charge generation for small ΔIE blends, which is mirrored by a strong and simultaneous field-dependence of the quantified photoluminescence from the NFA local singlet exciton (LE). We find that the decay of singlet excitons is the primary competition to free charge generation in low-offset NFA-based organic solar cells, with neither noticeable losses from charge-transfer (CT) decay nor evidence for LE–CT hybridization. In agreement with this conclusion, transient absorption spectroscopy consistently reveals that a smaller ΔIE slows the NFA exciton dissociation into free charges, albeit restorable by an electric field. Our experimental data align with Marcus theory calculations, supported by density functional theory simulations, for zero-field free charge generation and exciton decay efficiencies. We conclude that efficient photocurrent generation generally requires that the CT state is located below the LE, but that this restriction is lifted in systems with a small reorganization energy for charge transfer.A quantitative study, supported by Marcus theory and DFT, showing why the fate of singlet excitons is the pivot to free charge generation in low-energy offset organic solar cells.Fonds Wetenschappelijk Onderzoek 10.13039/501100003130European Research Council 10.13039/501100000781Deutsche Forschungsgemeinschaft 10.13039/501100001659China Scholarship Council 10.13039/50110000454