Photodetectors are of great interest in several technological applications thanks to their capability to convert an optical signal into an electrical one through light-matter interactions. In particular, broadband photodetectors are used in multiple applications such as environmental monitoring, imaging, fire detection, and astronomical observations. We present a two-dimensional photodiode heterojunction based on reduced graphene oxide (rGO) deposited on an n-type Silicon substrate. We report on the electro-optical properties of the device that have been measured in dark and light conditions into a spectral range from UV to IR. The room temperature current-voltage (I-V) measurements of rGO/n-Si photodetector exhibits a reverse saturation current linearly dependent on the light power. The main figures of merit of the photodetector such as linearity and responsivity have been evaluated and compared with the recent progress obtained substituting the rGO with a graphene single layer (Gr) on the similar n-Si substrate. The photoconductive properties and analysis of the two devices are presented and discussed. Finally, the experimental results demonstrate the feasibility of the rGO/n-Si and Gr/n-Si device to detect light from UV to IR light, nominating graphene-based heterojunction as a novel candidate for the realization of new broadband photodetectors.

Innovative applications in light signal sensing require flexibility, lightweight, low-cost and low temperature processability, as well as high sensitivity and quantitative photoresponse. We report on flexible organic photo transistors (OPTs) with excellent reproducibility in the detection of outstandingly low light intensities of few nW cm(-2). The optical response of the phototransistor was thoroughly characterized as a function of the wavelength, the incident optical intensity and the electrical polarizations. The photodetectors work reliably under bending and pulse cycling in different environmental conditions, at low polarizations down to the range 0.1-5 V. Based on these outcomes, arrays for broadband or multiple wavelength detection were implemented and successfully tested. The proposed device structure, derived from a layered p-p heterojunction, was developed to be easily implementable in each wavelength of interest. These results show a prospect of highly sensitive flexible OPT arrays for both experimental research and innovative optoelectronic systems.

Silicon heterojunction (HJ) solar cell is one of the leading technologies for single junction photovoltaic devices and it could be successfully integrated into a silicon based multijunction solar device, in conjunction with a large band gap cell. To be used as a bottom cell, the HJ device needs to adapt to the current technology for top cells, which are typically far less mature and characterized by smaller areas. However, if the HJ device is cut from a finished larger cell, the original edge passivation gets lost, causing the introduction of surface recombination paths and an overall decrease of the device performance. In this context, the development of a proper edge passivation procedure to be applied to high quality HJ solar cells becomes of major importance. We tried two different strategies of edge passivation based either on wet processes, such as the mesa etching of the device, or plasma treatment of the vertical edges. The latter consisting in the deposition of 1-10 nm of hydrogenated amorphous Si on the edges. In order to characterize the recombination losses and quantify the effect of different passivations, we did current-voltage measurements in light and dark conditions and quasisteady-state photoconductivity measurements.

Silicon heterojunctions (Si HJs) are regarded as a promising structure to develop high efficiency tandem solar cells. However, the Si HJ structure has been optimized for use as single junction device and variations might be made in order to adapt it to tandem applications. In this work we analyse by optical simulations the improvements that could be achieved in the short circuit current of Si HJs used as bottom cell in tandem devices, if the front TCO layer is made thicker. This solution allows to adapt the absorption in the Si HJ to the incident light spectrum, that is rich of high wavelength photons if a wide band gap cell is put on top of the Si HJ. The investigation has been carried out for two top cell technologies: a high efficiency InGaP solar cell and a semitransparent perovskite solar cell. The improvement in JSC is of the order of 1 mA in both cases, and is more pronounced when the edge of transparency of the top cell occurs at higher wavelength.

Chemical Vapor Deposited (CVD) graphene is an attractive candidate as transparent conductive electrode (TCE) for solar cells. Here it is proposed as TCE for silicon heterojunction solar cells (Si-HJSCs) and tested over devices with area up to 4 cm realized on n-type c-Si wafers with three different p-type emitters facing the dry-transferred graphene stack: amorphous silicon (a-Si:H), nanocrystalline silicon (nc-Si:H), and nanocrystalline silicon oxide (nc-SiO:H). A dependence of the cell performance on the material in contact with graphene has been observed with the most promising results in case of nc-Si:H and nc-SiO emitter. In particular, with respect to reference cells with standard TCE a short circuit current density gain of 1.4 mA/cm has been achieved when including an antireflection coating. The present work demonstrates the high quality of dry-transferred CVD-graphene over relatively large area and the feasibility of Si-HJSCs integrating graphene, where the fabrication sequence asks for a transfer process that guarantees a much better contact with the underlying functional layer with respect to more common designs where graphene is transferred onto a support substrate. This approach provides a possible pathway to Si-HJSCs free from conventional plasma-based TCEs and their deleterious effects due to plasma luminescence and ion bombardment, while offering an interesting platform for getting insights in the mechanisms at play at the interfaces with graphene within Si wafer-based devices.

We report the photocatalytic efficiency of CuO nanowires covered with a thin TiO2 film, studied by dyes degradation in water. The CuO nanowires were synthesized on Cu foils by thermal oxidation. A subsequent TiO2 deposition (7, 15, 30, 50 nm thick) was performed by atomic layer deposition, developing an ultrathin p-n heterojunction. A structural characterization was obtained by X-ray diffraction analysis, scanning and transmission electron microscopies equipped with energy dispersive x-ray analysis. The photocatalytic activity of the investigated materials was tested by the degradation of a cationic (methylene blue) or anionic (methyl orange). The relevance of the reported results was discussed in relation with the effects of the ultrathin p-n TiO2/CuO heterojunction. The two semiconductors are in intimate connection increasing the exposed surface and only TiO2 is directly in contact with water. This allowed to study systematically the effect of the electric filed generated by the p-n junction on the interface TiO2/liquid and therefore to modulate cationic/anionic dyes photo-degradation in water. (C) 2017 Elsevier B.V. All rights reserved.

Multiwall carbon nanotubes (MWCNTs) consist of multiple layers of graphite sheets arranged in concentric cylinders, from two to many tens. These systems are closely related to graphite layers but in some features, MWCNTs behave quite differently from graphite. In particular, their ability to generate a photocurrent in a wide wavelength range has been demonstrated either without or with the application of a draining voltage. In addition, the photocurrent signal has been found to reproduce the optical absorbance of MWCNTs, showing a maximum in the near UV region. In this paper main characteristics of a novel large area photodetector featuring low noise, high linearity and efficiency are reported. This detector has been obtained by coupling the optoelectronic characteristics of MWCNTs with the well-known properties of silicon. MWCNTs are growth on n-doped silicon layer by chemical vapour deposition creating a p-n heterojunction with high sensitivity to the radiation from UV to IR. An additional MIS junction is obtained with a metallic conductive layer deposited on the back of silicon substrate. Moreover, first results on the signals generated by pulsed laser are also reported.

Negative differential resistance (NDR), for which the current is a decreasing function of the voltage, has been observed in the current-voltage curves of several types of structures. We measured tunnelling current and NDR by illuminating large area heterojunction obtained by growing Multi Wall Carbon Nanotubes on the surface of n-doped Silicon substrate. In the absence of light, the current flow is null until a junction threshold of about 2.4 V is reached, beyond which the dark current flows at room temperature with a very low intensity of few nA. When illuminated, a current of tens nA is observed at a drain voltage of about 1.5 V. At higher voltage the current intensity decreases according to a negative resistance of the order of M?. In the following we report details of tunneling photodiode realized and negative resistance characteristics.

Hybrid organic/inorganic nanocomposites comprised of nanocrystalline iron oxide at the metastable beta-phase and graphitic carbon nitride (g-C3N4) were prepared via a facile in-situ growth strategy embedded in a solid state process. The hybridized beta-Fe2O3/g-C3N4 nanomaterials were thoroughly characterized by a variety of techniques, including UV-vis absorption, nitrogen physisorption, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and transmission electron microscopy (TEM). Their photocatalytic activity was evaluated under both simulated solar light and pure visible light irradiation against the photodegradation of methyl orange (MO), rhodamine B (RhB) and phenol. The prepared beta-Fe2O3/g-C3N4 nanocomposites were proven durable and significantly more efficient than the single components. The beta-Fe2O3 content in the final material was tuned to optimize the photocatalytic performance, with particular attention to the activity under visible light. The enhanced photoactivity was attributed to a) the improved optical properties of the prepared nanocomposites, presenting narrower band-gap energies and increased visible light absorption efficiency, and b) to the efficient separation of the photoinduced charge carriers driven by the matched band edges in the heterostructure. The predominant active species responsible for the photodegradation activity were determined and a possible mechanism is proposed. (C) 2016 Elsevier B.V. All rights reserved.

A significant resonant tunneling effect has been observed under the 2.4 V junction threshold in a large area, carbon nanotube-silicon (CNT-Si) heterojunction obtained by growing a continuous layer of multiwall carbon nanotubes on an n-doped silicon substrate. The multiwall carbon nanostructures were grown by a chemical vapor deposition (CVD) technique on a 60 nm thick, silicon nitride layer, deposited on an n-type Si substrate. The heterojunction characteristics were intensively studied on different substrates, resulting in high photoresponsivity with a large reverse photocurrent plateau. In this paper, we report on the photoresponsivity characteristics of the device, the heterojunction threshold and the tunnel-like effect observed as a function of applied voltage and excitation wavelength. The experiments are performed in the near-ultraviolet to near-infrared wavelength range. The high conversion efficiency of light radiation into photoelectrons observed with the presented layout allows the device to be used as a large area photodetector with very low, intrinsic dark current and noise.

This paper presents a comparative study of the influence of post-deposition annealing on amorphous silicon/crystalline silicon heterojunction solar cells deposited by ICP-CVD and PE-CVD techniques. Two major effects on the solar cell efficiency occur caused by thermal annealing. The first effect is a slight improvement of the performance on annealing up to 250 degrees C. The second effect, for annealing temperatures above 250 degrees C, reveals deterioration of the solar cell performance. It is suggested that both effects are related-to thermally activated diffusion of hydrogen. For low annealing temperatures, diffusion of weakly bonded hydrogen allows to passivate the defects in the amorphous emitter and at the heterointerface. In the high temperature annealing region, outdiffusion of hydrogen is assumed to be responsible for an increase of defect states in the structures. The results indicate a better stability after high temperature treatment for the sample prepared by ICP-CVD technology. (C) 2014 Elsevier B.V. All rights reserved.

Low temperature growth of GaAs (LT-GaAs) near 200 °C results in a recombination lifetime of nearly 1 ps, compared with approximately 1 ns for regular temperature ~600 °C grown GaAs (RT-GaAs), making it suitable for ultra high speed detection applications. However, LT-GaAs detectors usually suffer from low responsivity due to low carrier mobility. Here we report electro-optic sampling time response measurements of a detector that employs an AlGaAs heterojunction, a thin layer of LT-GaAs, a channel of RT-GaAs, and a vertical electric field that together facilitate collection of optically generated electrons while suppressing collection of lower mobility holes. Consequently, these devices have detection efficiency near that of RT-GaAs yet provide pulse widths nearly an order of magnitude faster-~6 ps for a cathode-anode separation of 1.3 ?m and ~12 ps for distances more than 3 ?m. © 2013 by the authors; licensee MDPI, Basel, Switzerland.

An important endeavor in modern materials science is the synthesis of adaptive assemblies with information processing capabilities similar to those of biological neural systems. Recent developments concern materials functionally similar to the memristor, a notional electrical circuit whose conductivity is dependent on past activity. This feature is analogous to synaptic plasticity: the ability of neurons to modify their synaptic connections as a result of accumulated experience-the basis of learning and the formation of memory. In this paper, we present the first evidence that memristive device-based organic materials show adaptive behavior similar to biological cognitive systems, using learning in the feeding neural network of the pond snail, Lymnaea stagnalis, as a specific biological reference. The synthetic reproduction of synaptic plasticity reported here can create new paradigms for novel computing systems and give impetus to the search for bio-inspired nanoscale molecular architectures capable of learning and decision making. © 2011 Springer Science+Business Media, LLC.

In this work we present the results we obtained by processing 1 ©cm p and n-type, multi-crystalline silicon wafers when a double hydrogenated amorphous /crystalline silicon heterojunction is applied to sandwich the substrate, at the sunward surface to form the emitter and at the rear side to act as back surface field. We have investigated the role of the multi-crystalline silicon surface conditioning, on the base of illuminated and dark current-voltage characteristic, quantum efficiency measurements, and overall photovoltaic solar cell performance. In particular, a comparison between an acidic isotexturing and a chemical polishing treatment is presented and discussed in detail. We have found that the morphology of the multicrystalline silicon surface plays a tough role in the a-Si:H properties when used as surface passivating layer. The commercial acidic surface treatment, commonly used in diffused junction mc-Si solar cell fabrication, requires a level of passivation which is not easily achievable using the very thin a-Si:H film, needed in heterojunction technology. The deposition of a uniform thickness over a whole textured silicon surface of a thin amorphous layers still requires further investigation and improvement. Nevertheless the high Voc obtained on p-type doped mc- Si (625 mV) remarks the effectiveness of a-Si:H/mc-Si technology in achieving high photovoltaic efficiency using low thermal budget manufacturing process.

Heterojunction solar cells are fabricated on 1 ?cm boron or phosphorus doped multicrystalline silicon wafers. As-cut material is treated by industrial isotexturing and home-made acidic texturing in order to obtain textured surfaces, as well as CP4 and modified CP6 chemical polishing, in order to obtain reference samples. A double a-Si:H/c-Si is deposited by Plasma Enhanced Chemical Vapour Deposition to sandwich the substrate, at the sunward surface to form the emitter and at the rear side to act as back surface field. High open-circuit current (625 mV) is obtained on flat p-type mc-Si, whereas commercial acidic surface treatment, commonly used in diffused junction mc-Si solar cell fabrication, requires a level of passivation which is not easily achievable using the very thin a-Si:H film needed in heterojunction technology. On n-type substrates, conversion efficiency around 12% is found, despite low VOC values. The effectiveness of a-Si:H/mc-Si technology in achieving high photovoltaic efficiency using low thermal budget manufacturing process is demonstrated.

Hydrogenated amorphous silicon/ CZ silicon heterojunction solar cells are fabricated by plasma enhanced chemical vapour deposition using an entirely low temperature process. Rear side deposition parameters, i.e. dopant gas phase concentration, depositon time, and silane dilution in hydrogen, are varied. Cell current-voltage characteristics under illumination, evidenced that a decrease of phosphine dilution in silane results in higher opencircuit voltage, due to lower defect concentration associated to lower doping. Furthermore, layer thickness is increased either by increasing deposition time or by increasing the silane/hydrogen ratio. In both cases, higher opencircuit voltage is obtained with thicker n+ layer, due to shift of the Fermi level towards the bulk value. In the case of high silane/hydrogen ratio, the amorphous to microcrystalline fraction of the rear layer is higher, thus partly acting as a back surface field.

We studied the passivation effect of plasma deposited intrinsic and doped hydrogenated amorphous silicon layers on industrial grade, n-type, 1 ?cm multicrystalline silicon, aimed at the fabrication of Heterojunction (HJ) solar cells based on such material. The investigated variables include the process temperature, hydrogen dilution of silane during the intrinsic layer deposition, and passivating layer thickness. The structure of the passivating layers was analysed by optical measurements. Surface photovoltage and quasi-steady state photoconductance were used to characterize untreated and treated wafers. Thin a-Si:H layers exhibited the best passivating performance.

We report the results on n/p heterojunction solar cells fabricated on textured CZ silicon. Both amorphous and microcrystalline emitters were fabricated using 13.56 MHz plasma enhanced chemical vapour deposition, and a maximum fabrication temperature of 250°C. All devices include an intrinsic buffer layer and a conductive antireflecting coating. An increase of efficiency from 10.5% to 11.8% was observed with respect to devices fabricated on flat substrates. The maximum short circuit current, 34.4 mA/cm2, was observed in the case of a microcrystalline emitter. The results are discussed, and some indications for further improvements are given.

Homo- and heterojunction silicon solar cells were grown by VHF-PECVD at low temperature. The deposition conditions of the intrinsic layers at the junction interface were varied to obtain epitaxial and amorphous Si buffer layers. The passivating properties of the epitaxial silicon, associated to lower absorption losses compared to amorphous silicon, are investigated. A comparison between amorphous / crystalline heterojunction solar cells and epitaxial c-Si devices shows that in the latter case the larger Jsc partially compensates the inferior passivating properties of the intrinsic epitaxial buffer layer. The Voc of the epitaxial devices is strongly affected by the hydrogen dilution of the gas mixture in the intrinsic buffer layer deposition, and increases up to 613 mV for the higher dilution used. The result is attributed to an improvement of the interface quality for growth conditions as close as possible to equilibrium. The recombination losses in the junction region are also investigated by the saturation current density measurements as a function of temperature. The best cell performance, Voc = 637 mV and 13.7% extrinsic efficiency on planar devices, is obtained in the case of an amorphous i-layer with a p-microcrystalline emitter.

The optimisation of the fabrication process of ?c-Si/c-Si heterojunction solar cells is discussed, in order to obtain high efficiency / low cost devices. The deposition of the various layers of the device, having p+-i-n-n+ structure, is carried out by Plasma Enhanced Chemical Vapour Deposition (PECVD) at Very High Frequency (VHF), with a process temperature as low as 170 °C. An hydrogen plasma treatment is also used as an alternative to the intrinsic layer for interface passivation. The front contact is obtained by a transparent conducting Indium Tin Oxide (ITO) layer, deposited by RF sputtering at 250°C, with an Al grid on top of it. This grid and the Al back contact are thermally evaporated. Particular attention was put in determining and minimising the contribution of each interface to series resistance. As an example, low temperature processing prevents from applying the standard metal alloying procedures used in silicon based microelectronics for contact fabrication. An alternative low temperature process for the rear contact formation is used, which gives a contact resistance lower than 0.03 Wcm2 . We studied each single interface, including the junction, by electrical characterisation. In this way, the interfaces affecting the series resistance can be identified and improved, and the technology needed for the production of ?c-Si / c-Si heterojunction solar cells can be accurately designed. Details of each process step and results obtained are discussed. An efficiency in excess of 13% was measured on test cells.