Plasmonics and Ultrafast NanoOptics

Solar Cell Research


Solar energy is expected to play a crucial role in the worldwide delivery of energy: by 2030 half of the present world’s energy needs is envisaged to be provided by the sun, a free, inexhaustible source that irradiate the globe with 6000 times the amount of our current rate of consumption.
Despite the fact that the idea behind the photovoltaic (PV) effect goes back to the 19^th century (Bequerel 1839) it took an extensive engineering effort to achieve a device which could directly convert radiation into electricity. Since then, PV cells have been improved to make them affordable and more efficient. However, classical crystalline silicon cells are still suffering from their extremely high manufacturing cost which is mainly due to the energy intensive chemical purification required.
A new generation of extremely promising solar cell has been identified in dye-sensitized solar cells (DSSCs), which has brought solar energy technology into a cheaper and more efficient market. Despite the fact that this technology has only been developed in the last 20 years, more than 300 patents have been filed on different aspects of DSSC technology. Power conversion efficiencies exceeding 11% have been already validated. Nonetheless, molecular engineering and  continuous advances in nanotechnologies offer novel strategies to reduce losses through recombination and extend the light absorption to longer wavelengths. This is, for example, achieved by the use of new dyes and by Quantum Dots as novel sensitizers. Furthermore, efficiencies can be raised by dedicated metallic nanostructures resulting in Plasmonic enhancements which virtually raise the effective cross section of the active layer and thus permit this layer to be much thinner. Apart from saving active material, this leads to a further reduction of losses through unwanted recombination.
Finally, a crucial point is the development of low temperature sintering procedures which permit the generation of flexible solar cells which can be embedded into non rigid systems, e.g. clothes.


Research Team Members

Dr Dominic Zerulla (PI)
Jennifer O'Reilly (PhD Student)
Codrin Andrei (PhD Student)
Éadaoin McClean (PhD Student)
Thomas O'Reilly (PhD Student)
Shashi Singam (PhD Student)

The team are strongly linked to a number of other research groups under an SFI funded research cluster entitled "Advanced Biomimetic Materials for Solar Energy Conversion", which links research groups from 3 Irish Universities, along with a number of industrial partners, to the common goal of improving techniques of harnessing the "free energy" of the sun to achieve sustainable energy production.


Research Topics

Temporal Response of DSSC's

The temporal response of a DSSC is critically linked to the intensity of the incident light. When a DSSC is partially illuminated and the incoming light is of low intensity, the response time of the cell is prolonged dramatically. In this research, the major components of the DSSC are investigated to find the source and to provide a model of the driving mechanisms behind this delay. Spectral analysis combined with time dependant voltammetry and temporal response analysis are used to understand how new dyes, while assembled in a DSSC, respond to light of different wavelength and intensity. This study shows a strong correlation between the light intensity, active area and excitation wavelength on the temporal response time of a DSSC.
This research provides a temporal response benchmark for comparing dye sensitised solar cells performance (in the same way as I/V or spectral analysis are employed) and can also be used to investigate further the ultra fast processes at the heart of the DSSC's. These cells use a mesoporous titanium dioxide scaffold, functionalised with an adsorbed dye, as the main active element and are typically regenerated with an iodide/tri-iodide electrolyte. The ultra fast processes of electron excitation and injection play a crucial role in this design. They have been investigated here with a view to establishing temporal analysis as a tool to characterise and assess the performance of the DSSC. The complete study of the temporal response timescales shows a cell response in the order of microseconds to tens of seconds.

Nanoscale Composition and Structure of DSSC's

The aim of this project is to develop novel materials and devices that mimic the natural photosynthesis process in plants with high power conversion efficiencies, the so-called dye sensitised solar cell (DSSC). The biomimetic DSSC is an emerging technology and assumes importance primarily due to its low-production cost and eco-friendly nature.
The research is oriented on the fabrication and characterization of novel DSSC materials for high efficiency light harvesting, using techniques emerging from bio-inspired nanotechnology. The aims of the project are: the investigation of the main physical mechanisms in a DSSC (i.e. electron generation, injection, transport and temporal response mechanism) and the development of architectures with desired photonic properties (i.e. efficient photo-response materials - photon absorbing antennas) with the final goal of creating novel cost-effective and lifetime-effective technology and transferring it from research to knowledge economy. Techniques employed in present include: AFM, SEM, FIB, EDX, WDS, XPS, XRD, EBSD, UV-Vis.


Quantum Dot Sensitised Solar Cells

The focus of this project is on the investigation of novel light harvesting nanomaterials which will be incorporated into photovoltaic devices. Nanomaterials of interest include core-shell quantum dots of a lead and zinc chalcogenide composition (chalcogen= sulphur, selenium, tellurium etc.), and cadmium chalcogenide based nanorods. Microscopic imaging techniques include: atomic force microscopy (AFM), photo-emission electron microscopy (PEEM), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and scanning nearfield optical microscopy (SNOM).

Quantum dot sensitised solar cells represent a new generation of solar cells that directly convert light into electricity, i.e.: photovoltaics. Q-dots are nano-sized particles, typically < 10nm in diameter, that have unique opto-electronic properties when compared to the bulk material. Optically, their absorption may be tuned to harness energy over the solar spectrum by altering the bandgap, Eg, of the Q-dot, and electrically they have reported multi-excitons or electron-hole pair generation (MEG) properties. MEG describes a single incoming photon of sufficient energy, e.g.: 2.5 Eg, producing more than one exciton. Therefore Q-dots incorporated in a solar cell have the potential to enhance the photovoltaic performance of a solar cell, i.e.: increased light to electricity conversion efficiencies. Lead sulphide (PbS) Q-dots are of interest due to its relatively low bulk bandgap of 0.41eV, allowing for a wide-tunable absorption in the visible to infrared range, i.e.: 400-1500nm, thereby matching and harvesting the majority of the solar radiation.
The performance and efficiency of a solar cell may be evaluated via electrical and optical techniques. These include: high resolution absorption spectrometry, FT-IR, state of the art electrometry; current-voltage analysis, transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and x-ray photoelectron spectrometry (XPS).

Plasmonic Enhanced Solar Cells

Surface Plasmons offer the possibility to confine or store electromagnetic radiation in the vicinity of a plasmon active layer (i.e. nanostructured metal / dielectric halfspace). In this project, computational and experimental investigations on the implementation of such a layer into the architecture of a DSSC  will be made. Specifically, the focus is to generate a broadband, polarisation and angle independent, efficient plasmon active layer; that can be cost efficiently integrated into an inverted DSSC.

The aim is to create a new route for the development of efficient solar cell technologies, which are sustainable and environmentally friendly. The main aspect of this project is to integrate sensitized solar cells (SSCs) with field enhancement techniques from the area of plasmonics. The strategy for the field enhancement is to excite Surface Plasmon Polaritons (SPPs) on dedicated nanostructures which will then lead to the required field distribution in the active solar cell layer. SPPs are electromagnetic surface waves propagating along the interface of a metal and a dielectric, essentially light waves that are trapped on the surface as a result of their interactions with the free electrons of the conductor. The project involves computer modelling of proposed structures, integrating said design into SSCs and ultimately optimising a design for large scale manufacturing.

Plasmonic Nanoplates for Enhanced Photovoltaic's

Plasmonic Triangular silver Nanoplates(TSNP) can offer a number advances for the enhancement of photovoltaic efficiencies. First the TSNP can be optically tuned to poorly absorbed solar spectrum regions to extend the solar light capture without the need for thicker active layers. Second the sub wavelength TSNP act as antennas which trap and couple solar wavelengths into the active layers extending and increasing the effective solar cell absorption cross section. Third the aspect ratio of the TSNP can be tuned to tailor the absorption / scattering cross section ratio in order to facilitate optical scattering or optical absorption modes within device architectures. Spectrally tunable TSNP plasmonic layers will be developed and investigate towards the enhancement and optimization of Dye Sensitized Solar Cells(DSSC). Electrolyte tailored TSNP will be used and developed including gold edge coated TSNP and silica coated TSNP. The techniques involve include UV-Vis, steady state and dynamic photoluminescene and photo induced absorption spectroscopies as well as scanning electron and atomic force microscopies. Using current-voltage characterisation under dark and standard solar illumination, current densities, will be determined and profiled across an extended range of the solar spectrum. A final significant milestone will be to measure increased photocurrents and device efficiencies in the presence of selected TSNP layers.

Novel Solar Cell Types

The high cost of photovoltaic systems versus low efficiency represents a challenge that needs to be addressed for the widespread application of this technology. The efficiency of p-n junction solar cells varies depending on the crystallinity and nature of materials employed, degree of contact between the junctions, the design of the device, and many other factors. In the last twenty years, along with implementation of silicon p-n junction solar cells, much work has been carried out in the field of non-silicon solar cell technology. Donor/acceptor molecules or polymers are expected to harvest light and through charge separation produce electrical current. Promising results have been achieved so far for hybrid solar cells such as dye sensitised solar cells [1], organic solar cells based on conducting polymer or small molecules, organic-inorganic hetero-junction solar cells, CdS/CdTe based solar cells, thin film solar cells, and flexible solar cells. However, efficiency of these devices can be improved by limiting charge recombination, increasing the amount of photons adsorbed and much more. These issues must be addressed in novel solar cells. In this regard, light trapping (LT) [2], use of cheaper dyes based on widely available metals with a wide red to near-infrared (NIR) absorption spectrum and high electron-donating properties [3] represent a feasible route to increase the efficiency of photovoltaics. Photo efficiency in thin-film silicon solar cells can also be improved by localized excitation of surface plasmons on small metal nanoparticles[4]. One of our goals is to optimise light conversion in thin-film solar cells by enhancing local surface plasmon of metal nanoparticles or tailored metal surfaces and by optical path of light (LT). Currently, a focus of our research is to find out how shape of nanoparticles and their distribution over the surface of a thin film solar cell can enhance efficiency in photovoltaics.

[1] a) M. Grätzel MRS BULLETIN 30, (2005), 23; b) M. Grätzel Accounts of Chemical Research, 42, (2009), 1788.
[2] S. Mokkapati et al. Applied Physics Letters 95, (2009), 53115.
[3] a) I. Himahori et al. Accounts of Chemical Research 42, (2009), 1809; b) J.J.Cid et al. Chem. Eur. J. 15, (2009), 5130.
[4] P.Q. Luo et al. Thin Solid Films 517, (2009), 6256.