Bifacial Solar Cells : High Efficiency Design, Characterization, Modules and Applications

Abstract

Economic growth is bound to energy consumption. Because anthropogenic emissions of carbon dioxide result primarily from the combustion of fossil fuels, energy consumption is at the center of the climate change debate.The goals of the European Union for 2020 were set: "the renewable energies shouldhave a 20% share of the total energy production and the overall CO2 emission should be reduced by 20%" [ERE].One way to achieve the proposed international goals is the use of photovoltaic solar energy. The growth rate of PV during 2011 reached almost 70%, an outstanding level among all renewable technologies [EPI]. This work is based on crystalline silicon and it explains the necessary processing steps to create a bifacial solar cell from a crystalline silicon substrate.Different structures for bifacial solar cells can be obtained using different processes. One of these structures was explained in detail, describing every step of the process tounderstand how the pn+, the pp+ junctions and the metal contacts are formed. It also explained the processing details to clean the silicon substrates before and after processing and most of the chemical reactions involved. The most decisive step in our process was the boron diffusion; it marked the difference to the standard solar cell process and it also represented the major challenge.After several attempts of process sequences and optimization of most fabrication steps, a final device was presented. It was published in 2010 by the author. The device can be manufactured with good reproducibility and reliability of the results in large quantities, within a small range of satisfactory efficiencies for the front side as well as for the rear side.We processed the first batch of 50 solar cells in 2008 obtaining 15.3% efficiency for the front side and 9.9% for the rear side. After the optimization of most processing steps, the best efficiency was 17.3% and 15% for the front and rear side of the device, respectively. The process was stable and could be extended up to 200 cells per batch. The total light induced degradation of the cells was less than 2% in average for the front and rear side, fulfilling the requirements for solar cell industry.A novel concept was applied for the first time to this type of device. The laser doping technique was tested over boron diffused substrates and solar cells were further processed. Combining our standard process with the laser doping, solar cell efficiencies of 18% for the front side and 17% for the rear side can be reached.An important challenge of special interest was the way these devices are measured. A solution and guideline was proposed to avoid mistakes when measuring bifacial solar cells and other light passing-through devices. We presented the most used characterization techniques for solar cells. These are I-V curve and spectral response. To characterize bifacial solar cells realistically it is necessary to consider the structure of the devices and their interaction with the measurement system. We identified and quantified the influence of different chuck reflectances for bifacialsolar cells, which can be up to 1% in short circuit current and efficiency.An alternative for bifacial measurement and quick classification of bifacial solar cells was presented. It allowed simultaneous front and rear illumination and gave more comprehensive information of the bifacial solar cell performance.It is important to mention that, there is no measuring standard yet for bifacial solar cells. We believe our study is a contribution to further define these standards. Selecting some of the finished bifacial solar cells we constructed bifacial solar modules, using different transparent rear foil sheets. Measurements using these modules were performed in Italy and in the city of Konstanz. Under several measurement conditions, the modules have exceeded our expectations of performance showing their maximum potential. Bifacial solar cells were laminated using a high reflecting rear foil. In this case the increase can be up to 8% comparing the cell before and after lamination. When we used a black foil for lamination, a reduction of about 3% relative in efficiency was observed.When measuring bifacial solar modules the reflectivity of the underlying surface plays an important role; we have measured an average increase of up to 7.9% (relative) of the bifacial mode compared to the monofacial mode. The highest increase was 20% in our experiment.The size of the underlying area exposed to direct sunlight plays also an important role. We have found that the usable area to reflect the light is 36 times the area of the module. To install this type of modules it is better to use a large separation from the ground and between modules, to take advantage of the reflections from the natural surfaces.Bifacial modules are versatile and they always present a higher performance compared to monofacial modules. The elevated performance of these modules will depend on the application they are used and the albedo amount they may collect. First applications of bifacial PV modules were for space. Later, the focus was the optimization of the albedo collection. Currently, the applications are very varied, being part of the architecture of a city and contributing to the energy supply.To conclude, a high efficiency design for bifacial solar cells was presented in thiswork. Cells were manufactured, measured and bifacial PV modules were further laminated. Indoor and outdoor measurement showed the great performance of this concept. Finally, some applications of these systems were presented.