Daniele Colonna

Interplay between transparency and efficiency in dye sensitized solar cells

In this paper we analyze the interplay between transparency and efficiency in dye sensitized solar cells by varying fabrication parameters such as the thickness of the nano-crystalline TiO(2) layer, the dye loading and the dye type. Both transparency and efficiency show a saturation trend when plotted versus dye loading. By introducing the transparency-efficiency plot, we show that the relation between transparency and efficiency is linear and is almost independent on the TiO(2) thickness for a certain thickness range. On the contrary, the relation between transparency and efficiency depends strongly on the type of the dye. Moreover, we show that co-sensitization techniques can be effectively used to access regions of the transparency-efficiency space that are forbidden for single dye sensitization. The relation found between transparency and efficiency (T&E) can be the general guide for optimization of Dye Solar Cells in building integration 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.

Thiazolo[5,4-d]thiazole-based organic sensitizers with strong visible light absorption for transparent, efficient and stable dye-sensitized solar cells

A small set of thiazolo[5,4-d]thiazole-based D–π–A organic dyes, endowed with bis-pentylpropylenedioxythiophene (ProDOT) moieties in the π-spacer, was designed with the aid of computational analysis, synthesized and characterized. The presence of bulky and electronrich ProDOT groups beside the electron poor thiazolothiazole unit induced optimal physico-chemical properties, including broad and intense visible light absorption. As a consequence, the dyes were particularly suitable for application in thin layer dye-sensitized solar cells (TiO2 thickness: 3.0–6.5 μm). Small-scale (0.25 cm2) devices prepared using standard materials and fabrication techniques gave power conversion efficiencies up to 7.71%, surpassing those obtained with two different reference dyes. Transparent larger area cells (3.6 cm2) also showed good η values up to 6.35%, not requiring the use of a co-adsorbent, and retained their initial efficiency over a period of 1000 h storage at 85 °C. These results make this new family of organic sensitizers promising candidates for successful application in the production of efficient and stable transparent DSSCs for building-integrated photovoltaics.

Bridged Phthalocyanine Systems for Sensitization of Nanocrystalline TiO 2 Films

Phthalocyanines based-dyes represent attractive alternatives to the expensive and polluting pyridyl based Ru complexes because of their photochemical and thermal stability, they do show in fact intense absorption in the UV/blue (Soret band) and the red/near IR (Q band) spectral regions and appear very promising as sensitizer dyes for DSSC. In this contribution we review the state of the art and the recent progress in the application of these materials as dyes for DSSC and present three new dyes which are bridged derivatives of Iron phthalocyanine. Synthesis, optical properties, electrochemical characterization and device performances are discussed with regard to the different substitution degree of the macrocycle.

Efficient Cosensitization Strategy for Dye-Sensitized Solar Cells

The challenge of increasing the photocurrent of a dye solar cell device by acting on the spectral response is approached herein. Cosensitization of nanocrystalline titania photoanodes by using two complementary dyes is investigated considering the dyeing time as an additional parameter for the optimization of the cosensitization process. We find that the characteristics of the cosensitized cell can outperform those of the cells made with each single dye. This effect is related to the reduction of the molecular stacking of one of the dyes, which quenches electron transfer to TiO2. Cosensitization results are also related to the cell transparency.

Dye Solar Cells: Basic and Photon Management Strategies

After the introduction in 1991 by B. O’Regan and M. Gratzel, Dye Solar Cells (DSCs) have reached power conversion efficiency values over small area device as high as 11%. Being manufactured with relatively easy fabrication processes often borrowed from the printing industry and utilizing low cost materials, DSC technology can be considered nowadays a proper candidate for a large scale production in industrial environment for commercial purposes. This scenario passes through some challenging issues which need to be addressed such as the set-up of a reliable, highly automated and cost-effective production line and the increase of large area panels performances, in terms of efficiency, stability and life-time of the devices. In this work, an overview of the most utilized DSCs materials and fabrication techniques are highlighted, and some of the most significant characterization methods are described. In this direction, different approaches used to improves devices performances are presented. In particular, several methods and techniques known as Light Management (LM) have been reported, based to the ability of the light to be confined most of the time in the cell structure. This behavior contributes to stimulate higher levels of charge generation by exploiting scattering and reflection effects. The use of diffusive scattering layers (SLs), nanovoids, photonic crystals (PCs), or photoanodes co-sensitization approaches consisting in the use of two Dyes absorbing in two different parts of the visible range, have been demonstrated to be effective strategies to carry out the highest values of device electrical parameters. Finally, to increase the light path inside the DSCs active layer, the use of refractive element on the topside (a complementary approach to SL) has been shown a promising possibility to further improve the generated photocurrent.

Bragg grating nanostructuring of the TiO2 layer in dye sensitized solar cells: an efficient method to enhance light harvesting

In this paper, we report an experimental procedure for active layer nanostructuring in Dye Sensitized Solar Cells (DSCs) to enhance light harvesting. A Bragg grating has been realized on a high performance commercial photoresist by means of the Laser Interference Lithography (LIL) technique. Subsequently this structure has been replicated by a Soft Lithographic process on a polydimethylsiloxane (PDMS) mold, which finally allowed the direct imprinting of the DSCs titania layer under UV illumination. Morphological analysis demonstrated a successful pattern transfer over a large area. Spectroscopic and photovoltaic measurements have been performed on nanostructured and traditional bare DSCs. In the spectral range 500–750 nm the patterned cell showed a lower transmission and reflection indicating that the grating acts efficiently as a light harvesting element. I–V and Incident Photon to Current Efficiency (IPCE) characterization showed an enhancement of 31% of the cell efficiency, confirming the effectiveness of this method.