Life cycle analysis of organic tandem solar cells: When are they warranted? Nieves Espinosa, Frederik C. Krebs n Department of Energy Conversion and Storage, Technical University of Denmark, Frederiksborgvej 399, DK-4000 Roskilde, Denmark article info Keywords: Organic photovoltaics Tandem solar cells Multi-junction solar cells Roll-to-roll Life-cycle assessment Energy payback time abstract 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 improve- ment 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. & 2013 Elsevier B.V. All rights reserved. 1. Introduction There is an increased focus on environmentally benign manu- facture of energy technologies and special focus is devoted to technologies with a low degree of cumulative energy demand (CED) as this is a prerequisite for sustained growth and survival in the future. The objective of covering a large part of the energy demand with low carbon emissive energies is ambitious in some countries. As an example, Denmark has adopted the target of having 100% of renewable energy by 2050, which means that a lot of effort and funding must be put into the development of these currently non-existing alternative energies. In addition the cost must be low since it is expected that the turnaround due to a relatively shorter operational service life will cause an increase in the costs of electricity. In spite of the fact that these new technologies, such as wind and solar energy, require a high degree of initial investment, when they start operating and competing on the market, they have the particular advantage that their marginal costs are zero [1]. The Danish energy strategy is aligned with the forecasted decrease for photovoltaic electricity costs, with the levelized cost of electricity (LCOE) from PV estimated to be 15 €ct/kWh by 2050 [2]. From the range of PV technologies, so far, only silicon (Si) based PV technologies would be applicable on a large scale. Thin-film PV technologies such as CIGS- or CdTe-based solar cells are not an option since there is not even a fraction of the needed indium or tellurium available in the earth crust to fully cover the future energy demand [3]. However, the energy demand for producing Si based solar cells is on the order of several thousand MJ per square metre [4,5]. In the other extreme the organic photovoltaic (OPV) family presents the lowest energy embedded and as a technology it is potentially cheaper globally for the reasons that an energy producing unit can be manufactured in a much shorter timespan Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2013.09.013 n Corresponding author. Tel.: þ45 46 77 47 99. E-mail address: frkr@dtu.dk (F.C. Krebs). Please cite this article as: N. Espinosa, F.C. Krebs, Life cycle analysis of organic tandem solar cells: When are they warranted? Solar Energy Materials and Solar Cells (2013), http://dx.doi.org/10.1016/j.solmat.2013.09.013i Solar Energy Materials & Solar Cells ∎ (∎∎∎∎) ∎∎∎–∎∎∎