Prediction of Photocurrent in Bifacial Photovoltaic Systems

Motivated by the shift of the photovoltaic (PV) industry towards bifacial solar modules, desert markets, and ever narrower profit margins, this thesis study is aimed at improving the estimation accuracy of the output of bifacial PV systems that are potentially exposed to soiling. The scope is limited to the estimation of photocurrent, the performance parameter that is the most strongly affected by bifaciality and soiling. The thesis consists of three parts with the following topics: ground surface reflectance, module soiling, and bifacial effective irradiance. As for the first topic, the thesis aims at providing tools for surface reflectance estimation in any use case. The model evaluation results that are based on a global database of reflectance measurements can be used to choose a reflectance model that is appropriate for the surface type of interest. On a global average, the results show that data-driven estimation reduces the mean absolute error of reflectance estimates by 20-40% compared to the literature values. The proposed geographically generalised parametrisations of reflectance models can be used to produce temporally variable reflectance estimates if measuring campaigns for local model calibration cannot be implemented. If they can, the thesis provides detailed guidelines for their optimal timing. The choice of the reflectance model can result in deviations of up to 2.5% in the energy yield estimated for a bifacial PV system. In the second part of the thesis, a new physical model for forecasting soiling losses is proposed. The model incorporates new methods e.g., to simulate dust deposition on tilted surfaces, to physically capture the impact of relative humidity on dust accumulation, and to estimate the hemispherical transmittance of dust for diffuse light sources. The model can be used based on the public data products of numerical weather prediction models without the need for soiling measurements. The model is validated against its alternatives and measurements made at a field experiment in the United Arab Emirates. The impact of the soiling loss model on yield estimation is found to be lower than that of the reflectance model, plus minus 1% at maximum considering usual weekly or biweekly cleaning intervals. Finally, the third part of the thesis proposes a new model for simulating the photocurrent of each solar cell of a bifacial PV power plant using a computationally efficient method. It comprises a new algorithmic design that makes it possible to make all the detailed geometric calculations once and move to the temporal domain only to estimate irradiance. The most important novelty of the proposed model is that it estimates the photocurrent contributions as a distribution of the different light sources. The model is validated against the measurements made in France and Italy. The results of the validation show that the model's current estimates are more accurate than those of the widely used "unlimited sheds" approach. The most important reasons for the better performance are the new model's capability to estimate the spatial variability of effective irradiance on the array surface and that of ground-incident irradiance around the system. As per the impact assessment, the new model was found to overestimate yield and underestimate electricity cost by 1% on average. The "unlimited sheds" approach, in turn, resulted in more variable bias. In the case of large arrays, the approach was found to overestimate yield by 4%.

Nanoscale Organic / Inorganic Hybrid Photovoltaic Devices

Thomas Dufaux

There is an increasing trend of using various types of nanostructures as components of solar cells. This highlights the need to gain a deeper understanding of the nanoscale interface in such devices. The present thesis aims to study individual photoactive junctions as the elementary functional unit of bulk photovoltaic devices, and exploit the gained knowledge toward further improving the solar cells' efficiency. Along this direction, Schottky-contacts to cadmium sulphide nanowires were thoroughly studied using scanning photocurrent microscopy (SPCM). In conjunction with theoretical simulations of the measured photocurrent profiles, it was found that charge carriers can be very efficiently extracted out of a few micrometer long nanowire segments below the metal contact. Moreover, the carrier combination rate in this section could be determined as a function of an applied bias or backgate voltage. These findings provide valuable clues about how electric fields are distributed within semiconductor nanowire-based devices. Moreover, SPCM was employed to explore the local photoresponse along graphene/CdS heterojunctions, representing a first step toward implementing graphene as an active acceptor material into solar cells. The short circuit current of such devices could be enhanced by two orders of magnitudes through chemical tailoring of the graphene-CdS interface. In addition, evidence was obtained that surface plasmon excitation in the metal contacts can make significant contributions to the generated photocurrent. This effect was exploited in the fabrication of novel surface plasmon detectors, which support different plasmon modes depending on the polarization direction of the incoming light. Finally, it was attempted to realize photovoltaic devices comprising a pn-junction implemented into a π-conjugated organic polymer. While the electric-field assisted exciton dissociation in this system could be successfully modelled in the framework of the Braun-Onsager model, photoinduced charge separation at the interface between differently gated regions of the polymer could not be achieved due to insufficient electron conduction of the material.

EPFL2012

Alternative redox systems for the dye-sensitized solar cell

Hervé Nusbaumer

Due to their high efficiencies and their potentially low production costs, dye-sensitized solar cells (DSSC) have attracted much attention during the last few years. The technology is based on a layer made of mesoscopic TiO2 film which significantly increases the optical path for light harvesting by the surface-anchored sensitizer molecules, whilst keeping an efficient contact with the electrolytic solution. These sensitizer molecules are often based on ruthenium polypyridyl complexes because of their high absorption coefficients in the visible, but any dye could be used to sensitize the semiconducting TiO2. The standard DSSC usually contain organic electrolytes with dissolved triiodide/iodide as the redox couple. Cells based on alternative electrolytes have been developed and were used in early DSSC designs, but showed somehow deceiving photovoltaic performances. Until now, endeavors to increase the photovoltage of dye-sensitized nanocrystalline cells by replacing iodide by a one-electron, outer-sphere redox couple with a more positive oxidation potential have not succeeded. This has been largely due to limitations imposed by the dark current and the slow diffusion of the alternate mediator. The work presented here focused on finding efficient alternative redox couples for the DSSC, able to rival the classical triiodide/iodide couple. The tested compounds were triarylamines, platinum complexes and cobalt complexes. Triarylamines and platinum complexes proved considerably less effective than the classical couple in term of photoelectrochemical efficiencies. Cobalt complexes however displayed far more interesting behavior. The experiments carried out demonstrated that the [Co(bip)2]3+/2+ complexes were the most suitable candidates to be used as redox mediators as their photoelectrochemical and kinetic behavior rivaled those of the triiodide/iodide redox couple. The system however had to be optimized to reach high efficiencies. Heteroleptic dyes incorporating long alkyl chains were found to be the most appropriate category of dye, boosting further the DSSC efficiency by reducing the dark current within the cells. Co-grafting the sensitizer and co-adsorbent led to an improved photovoltaic efficiency through the increased photocurrent. Silver addition in the dye solution also leads to a global improvement of the photovoltaic parameters. To avoid diffusion problems with the large molecules in the electrolyte, very thin nanocrystalline TiO2 layers had to be employed. A 3 μm thick nanoporous layer with a 3 μm scattering layer was found to be optimal. As cobalt complexes may undergo electrochemical processes on the SnO2 layer of the photoelectrode, a blocking layer of compact TiO2 was used to reduce the dark current. The electrochemical regeneration of the cobalt complexes with the SnO2 is not fast enough to support cell function and the counter electrode must be covered either with platinum or gold layer to ensure a good fill factor. The optimization of the electrolyte itself mainly concerned the choice of the solvent, the redox mediator concentration, the best degree of oxidation, the additives that can be used to improve the photovoltaic parameters and finally how to increase the redox couple solubility. The best efficiencies were achieved with an acetonitrile/ethylene carbonate (2:3) solvent mixture, with a total redox mediator concentration between 0.10 to 0.15 M. If 10% of this total concentration is oxidized, the current density reaches its maximum whilst having a negligible effect on the photovoltage. 4-tert-butylpyridine (TBP) and lithium perchlorate were tried as additives in the electrolyte, as TBP increases the photovoltage and Li+ the photocurrent. The optimal concentrations of these two additives was found to be around 0.05 M TBP and 0.1 M LiClO4 for an electrolyte having a total cobalt complex concentration of 0.1 M. Mass-transport within the cell is the limiting factor for high currents when using cobalt complexes as redox mediators. Attempts to reduce the solvent viscosity by using alternative solvents to the acetonitrile/ethylene carbonate mixture were not successful. One way to solve the diffusion problem was to irradiate the cell from the counter electrode side. This method works well at high irradiation intensities. Finally, reduction of the distance between the electrodes also enhances diffusion. The use of [Co(bip)2]3+/2+ complexes, with appropriate dyes and taking into account the optimizations already described above, allowed efficiencies of 8% under an irradiance of 95 W · m-2, as well as IPCEs over 80% at 540 nm to be obtained. At full sun, efficiencies of over 4% were reached, with short circuit current densities of 9.5 mA · cm-2 and open circuit voltages of the order of 860 mV.

EPFL2004

Optimisation of Urban Form by the Evaluation of the Solar Potential

Marylène Montavon

What will the cities of tomorrow look like? This is a crucial question because in the four and a half billion years of its existence, the Earth has never been so threatened, as it is today. Since 2008 half of the world's population has lived in towns – more than 70% in rich nations. Three quarters of CO2 emissions are produced in towns, 75% of the world's energy is consumed in towns. The battle for our climate, for the continuation of our species and all other life, will be largely waged in cities. Will it be possible to create the ideal ecological town? The objective of this study is to compare actual and theoretical urban forms in order to explore the diverse effects of daylighting and solar potential on densely built-up sites. Indeed, the search for ideal solar orientations has been a persistent problem from the 19th century until the beginning of the 20th. Even today, for all our scientific knowledge and empirical experience of solar energy, making the correct choice has still not crossed the boundary from an enigmatic, tricky art into the field of an exact science. As a first step, six existing urban sites in Switzerland and Brazil, as well as two projects in the United Kingdom, a few generic models and a utopian city, were modelled and analyzed. The goal was not merely the construction of new, exemplary districts but also the transformation of existing towns. The findings have been used to assess the potential of façades and roofs located in urban areas for active and passive solar heating, photovoltaic electricity generation and daylighting. The results obtained from these different case studies revealed large variations of the potential for solar energy collection on the buildings' façades and roofs. Ideally, these investigations are expected to help architects and town planners understand how to optimise solar collection on buildings design. In addition, a study was made of the Contemporary City of Three Million Inhabitants created by Le Corbusier. According to the latter, the Contemporary City could increase the urban capacity and at the same time improve the urban environment and the efficiency of the city. Therefore the present study extracted the solar potential of this utopian city in order to assess Le Corbusier's propositions. Since 1922, Le Corbusier's studies have influenced architects and town-planners in their choice of orientations and urban forms: it was therefore crucial to discover if this reliance on Le Corbusier was justified. Solar irradiation and illuminance values obtained through numerical simulations form the core part of the method. The method adopted for the entire set of computer simulations consisted in modelling — via a computer-aided drawing piece of software, namely AutoCAD — the digital model of each building or group of buildings prior to exporting them to an urban performance analysis software (PPF/RADIANCE software package). Spatial distributions of solar irradiation and daylight fluxes over the overall building façades and roofs were calculated using ray-tracing simulation techniques to determine the appropriate placement of different solar technologies (passive and active solar, photovoltaic and daylighting). Performance indicators were used to assess the solar utilisation potential of the urban sites (e.g. statistics of sky view factors and daylight factors) in order to determine the optimal solar strategies for a given urban context. In addition, various works and sun-related recommendations that have appeared mainly since the end of the nineteenth century, together with the various scientific models and software tools pertaining to this topic, are presented in this study.

EPFL2010