CVD-graphene/graphene flakes dual-films as advanced DSSC counter electrodes

The use of graphene-based electrodes is burgeoning in a wide range of applications, including solar cells, light emitting diodes, touch screens, field-effect transistors, photodetectors, sensors and energy storage systems. The success of such electrodes strongly depends on the implementation of effective production and processing methods for graphene. In this work, we take advantage of two different graphene production methods to design an advanced, conductive oxide- and platinum-free, graphene-based counter electrode for dye-sensitized solar cells (DSSCs). In particular, we exploit the combination of a graphene film, produced by chemical vapor deposition (CVD) (CVD-graphene), with few-layer graphene (FLG) flakes, produced by liquid phase exfoliation. The CVD-graphene is used as charge collector, while the FLG flakes, deposited atop by spray coating, act as catalyst for the reduction of the electrolyte redox couple (i.e., I3-/I-- and Co+2/+3). The as-produced counter electrodes are tested in both I3-/I-- and Co+2/+3-based semitransparent DSSCs, showing power conversion efficiencies of 2.1% and 5.09%, respectively, under 1 SUN illumination. At 0.1 SUN, Co+2/+3-based DSSCs achieve a power conversion efficiency as high as 6.87%. Our results demonstrate that the electrical, optical, chemical and catalytic properties of graphene-based dual films, designed by combining CVD-graphene and FLG flakes, are effective alternatives to FTO/Pt counter electrodes for DSSCs for both outdoor and indoor applications

Ion Migration‐Induced Amorphization and Phase Segregation as a Degradation Mechanism in Planar Perovskite Solar Cells

The operation of halide perovskite optoelectronic devices, including solar cells and LEDs, is strongly influenced by the mobility of ions comprising the crystal structure. This peculiarity is particularly true when considering the long‐term stability of devices. A detailed understanding of the ion migration‐driven degradation pathways is critical to design effective stabilization strategies. Nonetheless, despite substantial research in this first decade of perovskite photovoltaics, the long‐term effects of ion migration remain elusive due to the complex chemistry of lead halide perovskites. By linking materials chemistry to device optoelectronics, this study highlights that electrical bias‐induced perovskite amorphization and phase segregation is a crucial degradation mechanism in planar mixed halide perovskite solar cells. Depending on the biasing potential and the injected charge, halide segregation occurs, forming crystalline iodide‐rich domains, which govern light emission and participate in light absorption and photocurrent generation. Additionally, the loss of crystallinity limits charge collection efficiency and eventually degrades the device performance

Diffusion Length Mapping for Dye-Sensitized Solar Cells

The diffusion length (L) of photogenerated carriers in the nanoporous electrode is a key parameter that summarizes the collection efficiency behavior in dye-sensitized solar cells (DSCs). At present, there are few techniques able to spatially resolve L over the active area of the device. Most of them require contact patterning and, hence, are intrinsically destructive. Here, we present the first electron diffusion length mapping system for DSCs based on steady state incident photon to collected electron (IPCE) conversion efficiency ( η I P C E ) analysis. The measurement is conducted by acquiring complete transmittance ( T DSC ) and η I P C E spectra from the photo electrode (PE) and counter electrode (CE) for each spatial point in a raster scan manner. L ( x , y ) is obtained by a least square fitting of the IPCE ratio spectrum ( I P C E R = η I P C E -CE η I P C E -PE ). An advanced feature is the ability to acquire η I P C E spectra using low-intensity probe illumination under weakly-absorbed background light (625 nm) with the device biased close to open circuit voltage. These homogeneous conditions permit the linearization of the free electron continuity equation and, hence, to obtain the collection efficiency expressions ( η COL-PE and η COL-CE ). The influence of the parameter’s uncertainty has been quantified by a sensitivity study of L. The result has been validated by quantitatively comparing the average value of L map with the value estimated from electrochemical impedance spectroscopy (EIS)

Recent advances in organic dyes for application in dye-sensitized solar cells under Indoor lighting conditions

Among the emerging photovoltaic (PV) technologies, Dye-Sensitized Solar Cells (DSSCs) appear especially interesting in view of their potential for unconventional PV applications. In particular, DSSCs have been proven to provide excellent performances under indoor illumination, opening the way to their use in the field of low-power devices, such as wearable electronics and wireless sensor networks, including those relevant for application to the rapidly growing Internet of Things technology. Considering the low intensity of indoor light sources, efficient light capture constitutes a pivotal factor in optimizing cell efficiency. Consequently, the development of novel dyes exhibiting intense absorption within the visible range and light-harvesting properties well-matched with the emission spectra of the various light sources becomes indispensable. In this review, we will discuss the current state-of-the-art in the design, synthesis, and application of organic dyes as sensitizers for indoor DSSCs, focusing on the most recent results. We will start by examining the various classes of individual dyes reported to date for this application, organized by their structural features, highlighting their strengths and weaknesses. On the basis of this discussion, we will then draft some potential guidelines in an effort to help the design of this kind of sensitizer. Subsequently, we will describe some alternative approaches investigated to improve the light-harvesting properties of the cells, such as the co-sensitization strategy and the use of concerted companion dyes. Finally, the issue of measurement standardization will be introduced, and some considerations regarding the proper characterization methods of indoor PV systems and their differences compared to (simulated) outdoor conditions will be provided

On the role of PTB7-Th:[70]PCBM blend concentration in ortho-xylene on polymer solar cells performance

Low-band gap polymers as donors are the new approach to realize high efficiency polymer solar cells. Among them, PTB7-Th is one of the most promising candidates, thanks to the possibility of exploiting deposition protocols (technique, solvent, concentration) from the most-known PTB7, and demonstrating a power conversion efficiency exceeding 10%. In this paper, we focus on the role of the ink concentration for the film deposition and formation of the active layer. The effect of the solid content (PTB7-Th:[70]PCBM) in an ortho-xylene based solution on the polymer solar cell performance are investigated. Farther, a correlation between solid content and DIO additive in the ink formulation with respect to device performance is discussed. A remarkable PCE of 8.2% has been demonstrated

Perovskite photo-detectors (PVSK-PDs) for visible light communication

Visible Light Communication technology has the potential to become a leading actor in the future 5G network, thanks to features such as efficient “re-use of re-sources”, native privacy security and human safeness (compared to radio-frequency applications). Among the several configurations achievable to realize a VLC link, the use of innovative photovoltaic devices as receiver, based on organic-inorganic printable semiconductors (hybrid perovskite materials), has the potential to pave the way to new scenario applications, thanks to the inherent cheaper costs of fabrication via roll-to-roll solution-based processes, possibly on flexible substrates. In this context, we have fabricated photo-detectors with three different types of perovskite, and used with a commercial cool-white LED to realize a prototype of visible light communication data link. Using a single-stage trans-impedance amplifier for the perovskite photodetector, we have reached a remarkable frequency bandwidth close to ∼800KHz

A Polymer Bio–Photoelectrolytic Platform for Electrical Signal Measurement and for Light Modulation of Ion Fluxes and Proliferation in a Neuroblastoma Cell Line

Light control of living systems is an emerging field in bioelectronics, in regenerative medicine and cell‐based therapy. Herein, the design of a semitransparent bio–photoelectrolytic platform for control of a neuroblastoma cell line via light pulses is laid out. The platform is based on conjugated polymer films interfaced with a biological electrolyte solution confined in a compact chamber. Human SH‐SY5Y neuroblastoma cells are cultured for 3 days on the organic semiconductor and subjected to a pulsed light protocol. At the end of the culture time, proliferative activity of cells on the polymer film subjected to light pulses is reduced by 50% compared to the cultures kept in dark. An increase in intracellular Ca2+ level is observed, indicating a significant perturbation of the equilibrium potential of the cells. It is shown that the platform, in a sandwich‐type closed architecture with two transparent electrodes, can provide a tool for the initial recording of bioelectrical photovoltage signals (mV) that can complement analysis with more sophisticated electrophysiological tools. Obtained results can pave the way to new noninvasive photomanipulation techniques to stimulate/control living cells and their proliferation through both optical and electrical stimulation and probes, for application in the fields of biosensing and biomedicine

An Interlaboratory Study on the Stability of All‐Printable Hole Transport Material–Free Perovskite Solar Cells

Comparison between different laboratories on long‐term stability analyses of perovskite solar cells (PSCs) is still lacking in the literature. This work presents the results of an inter‐laboratory study carried out between 5 laboratories from 4 countries. Carbon‐based PSCs were prepared by screen printing, encapsulated and sent to different laboratories across Europe to assess their stability by the application of three ISOS aging protocols: (a) in the dark (ISOS‐D), (b) under simulated sunlight (ISOS‐L) and (c) outdoors (ISOS‐O). Over 1000 hours stability is reported for devices in the dark, both at room temperature and at 65 °C. Under continuous illumination at open circuit, cells survived only for few hours, although they recovered after being stored in the dark. Better stability is observed for cells biased at maximum power point under illumination. Finally, devices operate in outdoors for 30 days, with minor degradation, in two different locations (Barcelona, Spain and Paola, Malta). Our findings demonstrate that open circuit conditions are too severe for stability assessment and that the diurnal variation of the PV parameters reveals performance to be strongly limited by the fill factor, in the central hours of the day, due to the high series resistance of the carbon electrode