Daniele Benetti

Graphene below the percolation threshold in TiO2 for dye-sensitized solar cells

We demonstrate a fast and large area-scalable methodology for the fabrication of efficient dye sensitized solar cells (DSSCs) by simple addition of graphene micro-platelets to TiO2 nanoparticulate ...

Enhanced photovoltaic properties in dye sensitized solar cells by surface treatment of SnO2 photoanodes

We report the fabrication and testing of dye sensitized solar cells (DSSC) based on tin oxide (SnO2) particles of average size ~20 nm. Fluorine-doped tin oxide (FTO) conducting glass substrates were treated with TiOx or TiCl4 precursor solutions to create a blocking layer before tape casting the SnO2 mesoporous anode. In addition, SnO2 photoelectrodes were treated with the same precursor solutions to deposit a TiO2 passivating layer covering the SnO2 particles. We found that the modification enhances the short circuit current, open-circuit voltage and fill factor, leading to nearly 2-fold increase in power conversion efficiency, from 1.48% without any treatment, to 2.85% achieved with TiCl4 treatment. The superior photovoltaic performance of the DSSCs assembled with modified photoanode is attributed to enhanced electron lifetime and suppression of electron recombination to the electrolyte, as confirmed by electrochemical impedance spectroscopy (EIS) carried out under dark condition. These results indicate that modification of the FTO and SnO2 anode by titania can play a major role in maximizing the photo conversion efficiency.

Functionalized multi-wall carbon nanotubes/TiO2 composites as efficient photoanodes for dye sensitized solar cells

We report on the effects of incorporation of different concentrations of carboxyl group (COOH)-functionalized multi-wall carbon nanotubes (F-MWCNTs) into TiO2 active layers for dye-sensitized solar cells (DSSCs). Standard DSSCs with bare TiO2 exhibit a photo-conversion efficiency (PCE) of 6.05% and a short circuit current density (Jsc) of 13.3 mA cm−2. The presence of 2 wt% F-MWCNTs in the photoanodes increases the PCE up to 7.95% and Jsc up to 17.5 mA cm−2. The photoanodes were characterized using scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy. The electrochemical behaviour of the solar cells was investigated by electrochemical impedance spectroscopy (EIS). We attribute the improved performances to the combined effect of increased dye loading and reduced charge recombination (as clarified by dye loading and EIS measurements), due to the conformal coverage of F-MWCNTs, which allows fast and efficient charge collection in operating solar cells. These results can help in improving the PCE in DSSCs in an elegant and straightforward way, minimizing the need of additional steps (e.g. pre- and post-treatment with TiCl4) for photoanode preparation.

Near-infrared colloidal quantum dots for efficient and durable photoelectrochemical solar-driven hydrogen production

A new hybrid photoelectrochemical photoanode is developed to generate H2 from water. The anode is composed of a TiO2 mesoporous frame functionalized by colloidal core@shell quantum dots (QDs) followed by CdS and ZnS capping layers. Saturated photocurrent density as high as 11.2 mA cm−2 in a solar‐cell‐driven photoelectrochemical system using near‐infrared QDs is obtained.

Nanofiber-supported CuS nanoplatelets as high efficiency counter electrodes for quantum dot-based photoelectrochemical hydrogen production

We developed a hierarchically assembled hybrid counter electrode (CE) based on copper sulfide (CuS) nanoplatelets grown on polymer nanofibers. The resulting CE was used in a quantum dot (QD)-based photoelectrochemical (PEC) system for H2 generation in the presence of sacrificial agents (S2−/SO32−). The concept is to increase the specific surface area of the CE, aiming at maximizing charge exchange at the electrode, which boosts efficient generation of H2 and to obtain a stable structure for long-term operation of the device. Structural and morphological characterization indicated the presence of a covellite crystalline phase (CuS). PEC tests showed that the CuS nanoplatelets grown in the CEs could replace Pt CEs in either visible-active or near infrared (NIR)-active QD-based PEC systems. Specifically, saturation of the photocurrent density (∼7.5 mA cm−2) occurred at ∼0.6 V versus the RHE, when using a NIR QD-based TiO2 photoanode and a nanofiber-supported CuS as the CE. Stability tests of the nanofiber-supported CuS CE showed that 85% of the initial photocurrent density was maintained after ∼1 h, which is similar to that obtained with the Pt foil CE (86%). In contrast, CuS nanostructures directly deposited on FTO glass without nanofibers (CuS/FTO CE) exhibited poor stability. CuS/FTO CE degraded quickly, showing a 90% drop in the initial photocurrent within 200 s testing whereas a 14% drop in the initial photocurrent was observed for the CuxS on brass within 10 min of testing. Our new nanofiber supported-CuS CE stands out due to its higher performance compared to brass and its similar stability compared to Pt during long term PEC operation. Additionally, our hybrid CE showed a better catalytic performance than the Pt CE and good stability in cyclic voltammetry tests. These results demonstrate that the nanofiber-supported CuS is a promising cost effective alternative to Pt as a highly efficient CE for PEC H2 generation.

Ultrasmall PbS quantum dots: a facile and greener synthetic route and their high performance in luminescent solar concentrators

Synthesis of quantum dots (QDs) with widely size-tunable optical absorption and high photoluminescence quantum yield (PL QY) via a facile route is highly desired. By introducing tributylphosphine (TBP) into a relatively green synthesis method based on the use of S, PbCl2 and oleylamine (OLA), we conveniently synthesized ultrasmall PbS QDs with the first excitonic absorption peak wavelength as short as 705 nm, without using a glove box, which cannot be achieved by previously reported approaches, without involving smelly S precursors (such as bis(trimethylsilyl) sulfide). Such synthesized PbS QDs show narrow size distributions without any aggregation and demonstrate high PL QY in the range of 60–90%, depending on the QD size. Based on nuclear magnetic resonance spectroscopy and X-ray diffraction investigations, TBP was found to act as the passivation ligand on the surface of QDs while simultaneously assisting the transformation of PbCl2–OLA into more reactive Pb(OH)Cl that can directly participate the nucleation process, yielding ultrasmall PbS QDs. This new finding renders Pb(OH)Cl a very promising, new lead precursor for convenient synthesis of PbS and other lead-based QDs. We also demonstrate that the process can be readily scaled up. After synthesizing a thin CdS shell (∼0.1 nm), ultrasmall core/shell QDs with a large Stokes shift (0.36 eV) and good stability were employed for fabricating near infrared (NIR) luminescent solar concentrators, which led to a record-high optical efficiency of ∼1.2% at a geometric factor of ∼50 (10 cm in length). The TBP route developed herein is very promising for synthesizing high quality ultrasmall QDs that have high potential in NIR-related applications.

Combined magnetron sputtering and pulsed laser deposition of TiO 2 and BFCO thin films

We report the successful demonstration of a hybrid system that combines pulsed laser deposition (PLD) and magnetron sputtering (MS) to deposit high quality thin films. The PLD and MS simultaneously use the same target, leading to an enhanced deposition rate. The performance of this technique is demonstrated through the deposition of titanium dioxide and bismuth-based perovskite oxide Bi2FeCrO6 (BFCO) thin films on Si(100) and LaAlO3 (LAO) (100). These specific oxides were chosen due to their functionalities, such as multiferroic and photovoltaic properties (BFCO) and photocatalysis (TiO2). We compare films deposited by conventional PLD, MS and PLD combined with MS, and show that under all conditions the latter technique offers an increased deposition rate (+50%) and produces films denser (+20%) than those produced by MS or PLD alone, and without the large clusters found in the PLD-deposited films. Under optimized conditions, the hybrid technique produces films that are two times smoother than either technique alone.

Multiferroic Bi2FeCrO6 based p–i–n heterojunction photovoltaic devices

Photovoltaic devices made of ferroelectric films are being widely studied, due to their efficient charge separation driven by the internal polarization as well as above-bandgap generated photovoltages. These features may enable power conversion efficiencies (PCE) exceeding the Shockley–Queisser limit, which characterizes conventional semiconductor-based solar cells. However, improving the PCE of such devices is still a challenge, mainly due to the weak charge transport and collection induced by the recombination of photocarriers. Here, we fabricated p–i–n heterojunction devices based on double-perovskite multiferroic Bi2FeCrO6 thin films. The latter act as intrinsic absorbers, sandwiched between hole- and electron-transporting layers, a p-type NiO and an n-type Nb-doped SrTiO3 semiconductor, respectively. Under 1 sun illumination, the optimized p–i–n device yields an open-circuit voltage of 0.53 V and a short-circuit current density of 8.0 mA cm−2, leading to a PCE of ca. 2.0%, a four-fold enhancement compared to that of the i–n device architecture.

Direct Measurement of Electronic Band Structure in Single Quantum Dots of Metal Chalcogenide Composites

Metal chalcogenide quantum dots (QDs) are among the most promising materials as light harvesters in all-inorganic systems for applications in solar cells and production of solar fuels. The electronic band structure of composite QDs formed by lead and cadmium chalcogenides directly grafted on highly oriented pyrolytic graphite surfaces through successive ionic layer absorption and reaction is investigated. Atomic force microscopy and Kelvin probe force microscopy (KPFM) are applied to investigate PbS, CdS, and PbS/CdS QD systems. The variation of the surface potential of individual QDs is measured, investigating the evolution of the electronic band structure as a function of QD size and composition. A shift of the Fermi level toward more negative values occurs when QD size is increased. The shift is more pronounced in CdS than in PbS, while the composite PbS/CdS exhibits an intermediate behavior. The calculated shift is in good agreement with the experiments. These results highlight the ability of KPFM to directly measure the electronic band structure in individual QDs of metal chalcogenide composites. This feature regulates charge dynamics in composite systems, thereby affecting device performance. This work provides valuable insights for applications in several fields, in which charge injection plays a major role.

Solvent-Antisolvent Ambient Processed Large Grain Size Perovskite Thin Films for High-Performance Solar Cells

In recent years, hybrid organic-inorganic halide perovskites have been widely studied for the low-cost fabrication of a wide range of optoelectronic devices, including impressive perovskite-based solar cells. Amongst the key factors influencing the performance of these devices, recent efforts have focused on tailoring the granularity and microstructure of the perovskite films. Albeit, a cost-effective technique allowing to carefully control their microstructure in ambient environmental conditions has not been realized. We report on a solvent-antisolvent ambient processed CH3NH3PbI3−xClx based thin films using a simple and robust solvent engineering technique to achieve large grains (>5 µm) having excellent crystalline quality and surface coverage with very low pinhole density. Using optimized treatment (75% chlorobenzene and 25% ethanol), we achieve highly-compact perovskite films with 99.97% surface coverage to produce solar cells with power conversion efficiencies (PCEs) up-to 14.0%. In these planar solar cells, we find that the density and size of the pinholes are the dominant factors that affect their overall performances. This work provides a promising solvent treatment technique in ambient conditions and paves the way for further optimization of large area thin films and high performance perovskite solar cells.