Luigi Vesce

Efficient sintering of nanocrystalline titanium dioxide films for dye solar cells via raster scanning laser

By identifying the right combination of laser parameters, in particular the integrated laser fluence Φ, we fabricated dye solar cells (DSCs) with UV laser-sintered TiO2 films exhibiting a power conversion efficiency η=5.2%, the highest reported for laser-sintered devices. η is dramatically affected by Φ and a clear trend is reported. Significantly, DSCs fabricated by raster scanning the laser beam to sinter the TiO2 films are made as efficient as those with oven-sintered ones. These results, confirmed on three batches of cells, demonstrate the remarkable potential (noncontact, local, low cost, rapid, selective, and scalable) of scanning laser processing applied to DSC technology.

The role of printing techniques for large-area dye sensitized solar cells

The versatility of printing technologies and their intrinsic ability to outperform other techniques in large-area deposition gives scope to revolutionize the photovoltaic (PV) manufacturing field. Printing methods are commonly used in conventional silicon-based PVs to cover part of the production process. Screen printing techniques, for example, are applied to deposit electrical contacts on the silicon wafer. However, it is with the advent of third generation PVs that printing/coating techniques have been extensively used in almost all of the manufacturing processes. Among all the third generation PVs, dye sensitized solar cell (DSSC) technology has been developed up to commercialization levels. DSSCs and modules can be fabricated by adopting all of the main printing techniques on both rigid and flexible substrates. This allows an easy tuning of cell/module characteristics to the desired application. Transparency, colour, shape, layout and other DSSCs features can be easily varied by changing the printing parameters and paste/ink formulations used in the printing process. This review focuses on large-area printing/coating technologies for the fabrication of DSSCs devices. The most used and promising techniques are presented underlining the process parameters and applications.

Angular and prism coupling refractive enhancement in dye solar cells

We quantify the strong dependence of photocurrent on the angle of incidence of light in a dye solar cell (DSC). Under laser illumination the photocurrent increases for large incidence angles. The enhancements are different upon using or not a coupling prism. They are explained with a model including three different angular factors. The observed enhancements up to 25% can be useful for evaluating novel designs of an efficient photon management in DSCs. Even an effective refractive index neff≈2.0 for the mesoporous titania/electrolyte phase was retrieved from the angle dependent photocurrent.

Formulations and processing of nanocrystalline TiO2 films for the different requirements of plastic, metal and glass dye solar cell applications

We carried out a systematic study on the effect of nanocrystalline TiO2 paste formulations and temperature treatment on the performance of dye solar cells (DSCs) over a large temperature range, to provide useful information for the fabrication of both plastic and metal flexible devices. We compared conventional screen-printable and binder-free TiO2 pastes with a new formulation which includes hydroxylethyl cellulose (HEC), enabling the study of the effect of organic materials in the TiO2 layer in the whole 25-600 °C temperature range. Differently from the binder-free formulations where the device efficiency rose monotonically with temperature, the use of cellulose binders led to remarkably different trends depending on their pyrolysis and decomposition thresholds and solubility, especially at those temperatures compatible with plastic foils. Above 325 °C, where metal foil can be used as substrates, the efficiencies become similar to those of the binder-free paste due to effective binder decomposition and inter-nanoparticle bonding. Finally, we demonstrated, for the first time, that the simultaneous application of both temperature (110-150 °C) and pressure (100 MPa) can lead to a large improvement (33%) compared to the same mechanical compression method carried out at room temperature only.

Laser-Sintered

We have carried out a systematic and combined <i>I</i>- <i>V</i>, electrochemical impedance spectroscopy (EIS), and scanning emission microscopy (SEM) investigation of dye solar cells (DSCs) fabricated with laser-sintered TiO₂ photoanodes as a function of laser-integrated fluence Φ. We show that the electron lifetime τ_oc in the TiO₂ film extracted from EIS spectra monotonically increases with laser sintering fluence both at constant illumination and, even more significantly, at constant photoinduced charge <i>Q</i>_oc. The increase in τ_oc between the poorly sintered and the best laser-sintered devices is an order of magnitude at constant illumination (equivalent to the increase in device efficiency) and even greater at constant <i>Q</i>_oc. A strong correlation between τ_oc and the cell electrical parameters is observed, although device efficiency peaks with Φ whereas τ_oc does not. The rise of τ_oc with Φ indicates improved electromechanical bonding between TiO₂ nanoparticles due to the improved sintering carried out by the raster scanning laser process, as also confirmed by SEM images. The strong correlation between device performance and τ_oc indicates that laser processing may also have significant potential in many other device applications that require high-surface-area TiO₂ coupled with good electronic transduction.

\hbox{TiO}_{2}

Perovskite and dye-sensitized solar cells are PV technologies which hold promise for PV application. Arguably, the biggest issue facing these technologies is stability. The vast majority of studies have been limited to small area laboratory cells. Moisture, oxygen, UV light, thermal and electrical stresses are leading the degradation causes. There remains a shortage of stability investigations on large area devices, in particular modules. At the module level there exist particular challenges which can be different from those at the small cell level such as encapsulation (not only of the unit cells but of interconnections and contacts), non-uniformity of the layer stacks and unit cells, reverse bias stresses, which are important to investigate for technologies that aim for industrial acceptance. Herein we present a review of stability investigations published in the literature pertaining large area perovskite and dye-sensitized solar devices fabricated both on rigid (glass) and flexible substrates.

Films for Dye Solar Cell Fabrication: An Electrical, Morphological, and Electron Lifetime Investigation

Small area hybrid organometal halide perovskite based solar cells reached performances comparable to the multicrystalline silicon wafer cells. However, industrial applications require the scaling-up of devices to module-size. Here, we report the first fully laser-processed large area (14.5 cm2) perovskite solar module with an aperture ratio of 95% and a power conversion efficiency of 9.3%. To obtain this result, we carried out thorough analyses and optimization of three laser processing steps required to realize the serial interconnection of various cells. By analyzing the statistics of the fabricated modules, we show that the error committed over the projected interconnection dimensions is sufficiently low to permit even higher aperture ratios without additional efforts.

Stability issues pertaining large area perovskite and dye-sensitized solar cells and modules

Over the past few years, dye-sensitized solar cell (DSC) research has been focused on the material and process cost reduction, and on the electronics integration of the devices. Monolithic design is one of the most promising DSC architectures for mass production, because it allows the elimination of one conductive substrate and offers the possibility of printing layer-by-layer the materials that compose the structure. In this study, the formulation, the realization, and the processing of the spacer and the catalyst layers are proposed, and the relative performance in terms of J-V characteristics, incident photon to current conversion efficiency, and impedance analysis of the device with the optimized material thickness is reported. The optimized profile of the overall structure permits us to obtain masked cells with a conversion efficiency of about 5% with no chemical treatment.

Laser-Patterning Engineering for Perovskite Solar Modules With 95% Aperture Ratio

Scanning laser processing has become a useful and often used tool in thin film solar cell industries, since it enables precise, low cost, non-contact and highly automated fabrication processes such as scribing, patterning, marking, edge deletion, local melting and sintering. Dye Solar Cells (DSCs) are electrochemical photovoltaic devices representing an attractive technology for large area solar energy conversion since they utilize relatively cheap materials and simple manufacturing processes often “borrowed” from the printing industry. In this paper we show successful application of laser processing to this technology.