Field Effect versus Driving Force: Charge Generation in Small‐Molecule Organic Solar Cells

Efficient charge generation in organic semiconductors usually requires an interface with an energetic gradient between an electron donor and an electron acceptor in order to dissociate the photogenerated excitons. However, single-component organic solar cells based on chloroboron subnaphthalocyanine (SubNc) have been reported to provide considerable photocurrents despite the absence of an energy gradient at the interface with an acceptor. In this work, it is shown that this is not due to direct free carrier generation upon illumination of SubNc, but due to a field-assisted exciton dissociation mechanism specific to the device configuration. Subsequently, the implications of this effect in bilayer organic solar cells with SubNc as the donor are demonstrated, showing that the external and internal quantum efficiencies in such cells are independent of the donor-acceptor interface energetics. This previously unexplored mechanism results in efficient photocurrent generation even though the driving force is minimized and the open-circuit voltage is maximized

A Comparison of Charge Carrier Dynamics in Organic and Perovskite Solar Cells

The charge carrier dynamics in organic solar cells and organic–inorganic hybrid metal halide perovskite solar cells, two leading technologies in thin‐film photovoltaics, are compared. The similarities and differences in charge generation, charge separation, charge transport, charge collection, and charge recombination in these two technologies are discussed, linking these back to the intrinsic material properties of organic and perovskite semiconductors, and how these factors impact on photovoltaic device performance is elucidated. In particular, the impact of exciton binding energy, charge transfer states, bimolecular recombination, charge carrier transport, sub‐bandgap tail states, and surface recombination is evaluated, and the lessons learned from transient optical and optoelectronic measurements are discussed. This perspective thus highlights the key factors limiting device performance and rationalizes similarities and differences in design requirements between organic and perovskite solar cells

Enhancing the operational stability of unencapsulated perovskite solar cells through Cu–Ag bilayer electrode incorporation

We identify a facile strategy that significantly reduces electrode corrosion and device degradation in unencapsulated perovskite solar cells (PSCs) operating in ambient air. By employing Cu–Ag bilayer top electrode PSCs, we show enhanced operational lifetime compared with devices prepared from single metal (Al, Ag and Cu) analogues. Time-of-flight secondary ion mass spectrometry depth profiles indicate that the insertion of the thin layer of Cu (10 nm) below the Ag (100 nm) electrode significantly reduces diffusion of species originating in the perovskite active layer into the electron transport layer and electrode. X-ray diffraction (XRD) analysis reveals the mutually beneficial relationship between the bilayer metals, whereby the thermally evaporated Ag inhibits Cu oxidation and the Cu prevents interfacial reactions between the perovskite and Ag. The results here not only demonstrate a simple approach to prevent the electrode and device degradation that enhance lifetime and stability but also provide insight into ageing related ion migration and structural reorganisation

Biorenewable Solvents for High-Performance Organic Solar Cells

With the advent of nonfullerene acceptors (NFAs), organic photovoltaic (OPV) devices are now achieving high enough power conversion efficiencies (PCEs) for commercialization. However, these high performances rely on active layers processed from petroleum-based and toxic solvents, which are undesirable for mass manufacturing. Here, we demonstrate the use of biorenewable 2-methyltetrahydrofuran (2MeTHF) and cyclopentyl methyl ether (CPME) solvents to process donor: NFA-based OPVs with no additional additives in the active layer. Furthermore, to reduce the overall carbon footprint of the manufacturing cycle of the OPVs, we use polymeric donors that require a few synthetic steps for their synthesis, namely, PTQ10 and FO6-T, which are blended with the Y-series NFA Y12. High performance was achieved using 2MeTHF as the processing solvent, reaching PCEs of 14.5% and 11.4% for PTQ10:Y12 and FO6-T:Y12 blends, respectively. This work demonstrates the potential of using biorenewable solvents without additives for the processing of OPV active layers, opening the door to large-scale and green manufacturing of organic solar cells

Impact of Interface Energetic Alignment and Mobile Ions on Charge Carrier Accumulation and Extraction in p‐i‐n Perovskite Solar Cells

Understanding the kinetic competition between charge extraction and recombination, and how this is impacted by mobile ions, remains a key challenge in perovskite solar cells (PSCs). Here, this issue is addressed by combining operando photoluminescence (PL) measurements, which allow the measurement of real-time PL spectra during current–voltage (J–V) scans under 1-sun equivalent illumination, with the results of drift-diffusion simulations. This operando PL analysis allows direct comparison between the internal performance (recombination currents and quasi-Fermi-level-splitting (QFLS)) and the external performance (J–V) of a PSC during operation. Analyses of four PSCs with different electron transport materials (ETMs) quantify how a deeper ETM LUMO induces greater interfacial recombination, while a shallower LUMO impedes charge extraction. Furthermore, it is found that a low ETM mobility leads to charge accumulation in the perovskite under short-circuit conditions. However, thisalone cannot explain the remarkably high short-circuit QFLS of over 1 eV which is observed in all devices. Instead, drift-diffusion simulations allow this effect to be assigned to the presence of mobile ions which screen the internal electric field at short-circuit and lead to a reduction in the short-circuit current density by over 2 mA cm−2 in the best device

Negative correlation in neural systems

In our attempt to understand neural systems, it is useful to identify statistical principles that may be beneficial in neural information processing, outline how these principles may work in theory, and demonstrate the benefits through computational modelling and simulation. Negative correlation is one such principle, and is the subject of this work. The main body of the work falls into three parts. The first part demonstrates the space filling and accelerated central limit convergence benefits of negative correlation, both generally and in the specific neural context of V1 receptive fields. I outline two new algorithms combining traditional ICA with a correlation objective function. Correlated component analysis seeks components with a given correlation matrix, while correlated basis analysis seeks basis functions with a given correlation matrix. The benefits of recovering components and basis functions with negative correlations are shown. The second part looks at the functional role of negative correlation for integrate- and-fire neurons in the context of suprathreshold stochastic resonance, for neurons receiving Poisson inputs modelled by a diffusion approximation. I show how the SSR effect can be seen in networks of spiking neurons, and further show how correlation can be used to control the noise level, and that optimal information transmission occurs for negatively correlated inputs when parameters take biophysically plausible values. The final part examines the question of how negative correlation may be implemented in the context of small networks of spiking neurons. Networks of integrate-and-fire neurons with and without lateral inhibitory connections are tested, and the networks with the inhibitory connections are found to perform better and show negatively correlated firing patterns. This result is extended to more biophysically detailed neuron and synapse models, highlighting the robust nature of the mechanism. Finally, the mechanism is explained as a threshold-unit approximation to non-threshold maximum likelihood signal/noise decomposition

Cyclic sulphur inides and thiazenes: synthetic studies and some structural correlations

This thesis describes the work carried out by the author between September 197^ and September 1977. The shapes of known sulphur-nitrogen species are rationalised by linear interpolation and the probable structures of sulphur- nitrogen species, as yet uncharacterised, are discussed. Correlations between sulphur-nitrogen and sulphur-oxygen stretching frequencies and bond lengths are presented and predictions of shapes from infrared data are discussed. Preparation of derivatives of sulphur chlorides using methyleneamino-lithium proved to be an excellent route to old and new sulphur-nitrogen compounds. Reactions of (SNCl)(_3) were studied with a view to reaction mechanisms involved and the first fully characterised derivatives of 3-phenylcyclodithiadiazolium chloride were prepared. The reactions of sulphur-nitrogen-oxygen species especially hexaoxocyclotrithiazenide (3-) anions are reported including the preparation of new salts. Attempts were made to prepare new members of the "electron rich" aromatic series of sulphur nitrogen compounds. Although unsuccessful in the original aim, the new compounds S(_7)NCOCF(_3), (S(_5)N(_5)) (SnCl(_5)(OPCl(_3))) and S(_4)N(_4).POCl(_3).SnCl(_4) were prepared and the use of phosphoryl chloride as a solvent was investigated. The crystal structure of (SnCl(_5)(OPCl(_3))) is discussed and a series of CNDO/2 calculations with and without "d" orbital basis sets have been carried out on a series of "aromatic" sulphur-nitrogen compounds