Low-temperature, solution-deposited metal chalcogenide films as highly efficient counter electrodes for sensitized solar cells

Transition metal chalcogenide crystalline films FeSe2, Cu1.8S, and CuSe have been deposited from solution by drop casting their dissolved inks onto a conductive substrate, followed by a mild thermal treatment. We demonstrate that the resulting chalcogenide films exhibit an excellent catalytic activity and function as highly efficient counter electrodes (CEs) for dye-and quantum dot-sensitized solar cells (DSCs and QDSCs). In particular, the FeSe2 and CuSe films produced herein with novel morphologies show better catalytic activity than that of the conventional Pt coated CE used in DSCs and Cu2S in QDSCs, respectively. Ensuing devices present an improved photovoltaic performance with maximum values of 9.10% for DSCs and 4.94% for QDSCs, comparable to those based on Pt and Cu2S CEs. The efficient CE materials developed here from such a facile and scalable route offer strong potential for a broader solar cell application that requires low-cost and large-scale production.

Interface engineering in solid-state dye-sensitized solar cells

Jessica Krüger

Dye-sensitised nanocrystalline solar cells are currently subject of intense research in the framework of renewable energies as a low-cost photovoltaic device. In particular dye-sensitised cells based on spiro-MeOTAD have gained attention as promising approach towards an organic solid-state solar cell. However, the efficiency in such dye-sensitised solid-state solar devices (SSD) was so far only ca. 10 % of the values reported at AM1.5 for the classical dye-sensitised solar cell with an electrolyte hole transporting medium (DSSC). The objective of the present work is to study the limitations that emerge from the exchange of the electrolyte by the solid-state system and that oppose photovoltaic photon-to-electron conversion as high as for the DSSC. Interfacial charge recombination is an important loss mechanism in dye-sensitised solar cells. This is particularly true for SSD, as the solid hole-transporting medium is less efficient in screening of internal fields which assist recombination. A variety of strategies were tested in the SSD to minimise interfacial charge recombination. The most promising approach was the blending of the hole-transporting medium with tert.-butylpyridine (tBP) and lithium ions. Optical and electrochemical techniques, such as nanosecond laser spectroscopy, impedance spectroscopy and photovoltaic characterisation measurements, were used to study the impact of the additives on the SSD. Both lithium ions as well as tBP were found to increase the open circuit potential of the SSD. At the same time tBP was found to considerably lower the current output. The interaction of the additives was studied and their concentration in the spiro-MeOTAD medium optimised. The doping of the spiro-MeOTAD film, which was intended to support the hole transport, was found to enhance interfacial recombination significantly. The morphological properties of the TiO2, in particular layer thickness, particle size and film porosity, play a more important role in the SSD than in the DSSC. Penetration of the hole conductor into the TiO2 pores and electron diffusion length are coupled to these properties. As a result the light harvesting cannot be controlled at will via the TiO2 film thickness and the active surface area for dye adsorption. An enhanced light harvesting for thin TiO2 layers offers advantages for the charge transport and the formation of the interpenetrating network. The dye uptake in presence of silver ions was found to increase the dye loading and to significantly improve the device performance of thin TiO2 layer devices. The mechanism of this simple dye modification technique was studied by a variety of spectroscopic techniques. From spectroscopic evidence it is inferred that the silver is binding to the sensitiser via the ambidentate thiocyanate, allowing the formation of ligand-bridged dye complexes. The beneficial influence of the silver ions on the photovoltaic performance was not limited to the application of the standard N3 dye nor to the spiro-MeOTAD. SSDs were furthermore studied by frequency resolved techniques. Intensity modulated photocurrent spectroscopy (IMPS) and intensity modulated photovoltage spectroscopy (IMVS) were performed over a wide range of illumination intensities. The IMPS and IMVS responses provide information about charge transport and electron-hole recombination processes respectively. For the range of light intensities investigated, the dynamic photocurrent response appears to be limited by the transport of electrons in the nanocrystalline TiO2 film rather than by the transport of holes in the spiro-MeOTAD. The diffusion length of electrons in the TiO2 was found to be 4.4 μm. This value was almost independent of the light intensity as a consequence of the fact that the electron diffusion coefficient and the rate constant for electron-hole recombination both increase in the same way with light intensity but with opposite sign. The results of this work provide for a substantial improvement of the overall photovoltaic performance compared to earlier results for this type of SSD. However, this study reveals also that high conversion efficiencies as are measured for DSSC are not likely to be reachable with the spiro-MeOTAD system due to the significantly slower charge transport in the spiro-MeOTAD compared to the electrolyte redox mediator.

EPFL2003

Novel organic dyes for enhanced red to near IR response in dye sensitised solar cells

The aim of this work was to develop new, organic dyes, capable of harvesting photons in the red to near-IR region of the spectrum. A series of squaraine and tricarbocyanine dyes were produced and characterised and their optical and redox properties studied. The results of initial photovoltaic tests, followed by optimisations of cell performance, have also been presented. These clearly demonstrate the effectiveness of these dyes in solar cells. A total of 12 new squaraine dyes were successfully synthesised, seven of which were tested in "dye-sensitised" solar cells. The efficiencies ranged from 1.2%-3.7%, for the best dye [B1]. It was concluded that enhanced performance could be achieved in the presence of chenodeoxycholic acid (CDCA) and tetrabutylpyridine (TBP). These additives both help to prevent aggregation and increase the cell potential, limiting charge collection losses and enhancing performance. Three new tricarbocyanine dyes were also synthesised, two of which were used in solar cells. Unfortunately, due to the interaction of [B12] with the electrolyte, the dye proved to be unstable, as both the absorption maxima in UV-vis and fluorescence emission wavelengths were shifted to higher energies. Thus, the red response of the dye was significantly reduced and photovoltaic performance hindered. A limited photo-response of [B14] was also seen, although this dye appeared to be very stable in the presence of the electrolyte. To deduce the possible reasons for the poor performance of these dyes, their optical and electrochemical properties were investigated. Thus, cyclic voltametry was employed to identify the redox potentials of the dyes and their charge injection and recombination dynamics were studied through nanosecond time-resolved transient laser spectroscopy. It was concluded that the dyes theoretically possess the ideal characteristics for solar cell function and that the problem of [B14] lay in the purity of the dye. [B14] proved to be very difficult to purify as the dye was unstable in solution and decomposed on the columns employed in chromatography. Fortunately the dye appeared to be stable in the presence of an electrolyte when adsorbed on TiO2. The dye gave an IPCE of 12% between 800 – 900nm, which is particularly impressive, especially if the purity of the dye is considered. A new method of purification must be found however, if photovoltaic performance is to be enhanced. Finally, the effects of a compact blocking layer on device performance were studied. It was clearly demonstrated that a compact layer was essential. This was investigated and a relationship between the structure of the dye and the necessity of the blocking layer was established.

EPFL2006

Computational Modeling of Thin-Film Silicon Solar Cells and Modules

Thomas Andreas Lanz

Solar photovoltaic energy production relies on the direct conversion of sunlight into electricity. Silicon thin-film solar cells represent a reliable and environmentally friendly technology to make photovoltaics economically viable. As these solar cells do not contain any rare or toxic materials they are an ideal candidate for the large scale deployment of solar energy. At the same time photovoltaic energy production is ideal for off-grid, stand-alone installations. This thesis presents new methods and results on computational engineering of thin-film silicon solar cells and modules. Computational engineering employs numerical methods and simulation techniques to address engineering challenges. Predictive numerical simulations allow for device optimizations that otherwise require substantial experimental effort and they may lead to new insights into the device physics which is the basis for further improvements in terms of cell efficiency and lifetime. We discuss the development of two new simulation methods and present several case studies that illustrate their use in solar cell characterization and optimization. The first presented method addresses optical simulations, i.e. the interaction of the solar cell with sunlight. Our optical model is based on the net-radiation method that considers incoherent, ray-like propagation of light. We extend the net-radiation method with thin-film optics to allow for arbitrary sequences of thick and thin layers. Experimentally, advanced techniques such as optimized surface textures have been developed to increase the absorption in the solar cell and thereby to increase the produced current. We illustrate how our optical model takes into account these textures by means of azimuth integrated light scattering properties to formulate a computationally efficient model of light propagation within the solar cell. Our optical model thus integrates coherent and incoherent propagation of light as well as a non-iterative treatment of ligth scattering. It bridges the gap between wave and ray optics that alternative approaches are most often facing. We first apply the method to study a recently proposed rear electrode architecture based on lithium fluoride and aluminum in amorphous silicon solar cells. Our model-based optical analysis identifies the increased reflectivity of this back electrode as its main advantage. Secondly, we then explore the potential performance gains in microcrystalline silicon solar cells that can be reached by eliminating absorption in layers that do not contribute to the photogeneration of solar cell current. Finally, our optical model is also suited for other solar cell technologies and we illustrate its extension and application to light extraction calculations in light emitting diodes (LEDs). Our optical model has been incorporated into the software package setfos, commercialized by Fluxim Inc. It is thus available to the research community. We then turn to address large area solar modules. A new method based on the finite element method (FEM) is presented to calculate the electrical charge and heat transport in these devices and how it is influenced by defects as well as partial shading. We identify such defects with surface temperature measurements by means of lock-in thermography and demonstrate how our model accounts for the thermal signature of localized defects. Taking into account the large aspect ratio of thin-film solar modules we show how geometrical projections can be used to reduce the model complexity, leading to a numerically efficient approach based on the finite element method. We demonstrate this by presenting full current-voltage curves of solar modules under partial shading.

EPFL2013