1 Department of Energy Conversion and Storage, Technical University of Denmark2 Functional organic materials, Department of Energy Conversion and Storage, Technical University of Denmark3 Risø National Laboratory for Sustainable Energy, Technical University of Denmark
One approach to use solar radiation more effectively in solar cells is to stack, in series, multiple photoactive layers with complementary absorption spectra. Such devices are often termed tandem or multi-junction solar cells. The larger number of different materials and processing steps involved in their making when compared with the single junction solar cell has to be justified and compensated by a higher efficiency. A central question to ask is how much energy you need to invest in a system in order for it to produce energy and return the investment at least once and preferably a number of times. As an initial investigation into the potential viability of the tandem or multi-junction approach we have engaged in a detailed analysis based on the manufacturing energy for each step within the tandem module supply chain for full ambient processing of thin flexible polymer tandem solar cells prepared entirely by roll processing methods. We present a comprehensive overview of relevant research results on how the energy consumption affects the energy balance when using single and multi-junction solar cells. Based on the above question we calculate the minimum efficiency that the tandem or multi-junction should present to determine the minimum energy payback time; that is whether (or when) the increase in materials use and complexity of the tandem architecture is compensated by better performance. After analysing the performance and the consideration of a series of technical improvement opportunities, we project that the tandem solar cell has to be ~20% higher performing than the corresponding single junction solar cell to be warranted. We also highlight that there is a range in the reciprocal EBPT–efficiency relationship where the tandem solar cell is an advantage. Specific to polymer and organic solar cells are however that they embody very little energy and this implies that the single junction may be an advantage, especially in cases where land mass is not critical.
Solar Energy Materials and Solar Cells, 2014, Vol 120, Issue Part B
Organic photovoltaics; Tandem solar cells; Multi-junction solar cells; Roll-to-roll; Life-cycle assessment; Energy payback time