On the road to more efficient solar cells
Members of the TOPSICLE research team drew up a road map to facilitate the industrial scale production of 20% efficient mc-Si photovoltaic (PV) modules. The road map outlined two different cell concepts, the screen-printed cell and the buried contact cell. Project partners from the University of Konstanz in Germany carried out a detailed loss analysis and comparison of 17.6% and 18.1% mc-Si efficiency cells. The cells' efficiency potential was evaluated through the use of PC1D modelling with the aim of achieving large area mc-Si 20% solar cells in the future. Loss analysis for the 18.1% solar cell indicated that significant improvements could be realised by replacing the full area aluminium back surface field with a local rear contacting scheme. This change ensured a low back surface recombination velocity and high optical reflectivity for the rear surface. An additional thin thermal silicon oxide layer was placed beneath the silicon nitride antireflection coating in order to reduce the front surface recombination velocity. A key parameter in determining the performance of a solar cell is the fill factor. The addition of the silicon oxide layer resulted in a fill factor of 77% for the 18.1% cell. A fill factor of 78% had previously been achieved using a similar plating process and a factor of 79% may be considered realistic in the near future. A further improvement towards high efficiency can be made through the use of zero-shading loss cell design. This technique is based on the angled buried contact concept. The successful application of all the improvements prescribed by the roadmap could result in a current density of 39.6mA/cm2 and an open circuit voltage of 642.9mV. This would enable a 20.1% efficient mc-Si solar cell to be developed.
