www.MaterialsViews.com www.advenergymat.de FULL PAPER © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim (1 of 7) 1400084 wileyonlinelibrary.com Environmentally Printing Efficient Organic Tandem Solar Cells with High Fill Factors: A Guideline Towards 20% Power Conversion Efficiency Ning Li,* Derya Baran, George D. Spyropoulos, Hong Zhang, Stephane Berny, Mathieu Turbiez, Tayebeh Ameri, Frederik C. Krebs, and Christoph J. Brabec DOI: 10.1002/aenm.201400084 1. Introduction Organic photovoltaics (OPV) attracted increasing research interest in the last decade owing to its unique advantages over its inorganic counterpart, such as low cost, lightweight and easy manufacturing on a large-scale. [1–4] The power conversion The tandem concept involves stacking two or more cells with complementary absorption spectra in series or parallel connection, harvesting photons at the highest possible potential. It is strongly suggested that the roll-to-roll produc- tion of organic solar cells will employ the tandem concept to enhance the power conversion efficiency (PCE). However, due to the undeveloped deposi- tion techniques, the challenges in ink formulation as well as the lack of com- mercially available high performance active materials, roll-to-roll fabrication of highly efficient organic tandem solar cells currently presents a major chal- lenge. The reported high PCE values from lab-scale spin-coated devices are, of course, representative, but not helpful for commercialization. Here, organic tandem solar cells with exceptionally high fill factors and PCE values of 7.66% (on glass) and 5.56% (on flexible substrate), which are the highest values for the solution-processed tandem solar cells fabricated by a mass-production compatible coating technique under ambient conditions, are demonstrated. To predict the highest possible performance of tandem solar cells, optical simulation based on experimentally feasible values is performed. A maximum PCE of 21% is theoretically achievable for an organic tandem solar cell based on the optimized bandgaps and achieved fill factors. N. Li, D. Baran, H. Zhang, Dr. T. Ameri, Prof. C. J. Brabec Institute of Materials for Electronics and Energy Technology (i-MEET) Friedrich- Alexander-University Erlangen-Nuremberg Martensstraße 7, 91058 Erlangen, Germany E-mail: ning.li@fau.de G. D. Spyropoulos, Prof. C. J. Brabec Bavarian Center for Applied Energy Research (ZAE Bayern) Haberstraße 2a, 91058 Erlangen, Germany H. Zhang Erlangen Graduate School in Advanced Optical Technologies (SAOT) Paul-Gordan-Straße 6 91052, Erlangen, Germany Dr. S. Berny Merck Chemicals Ltd., Chilworth Technical Centre University Parkway Southampton SO16 7QD, UK Dr. M. Turbiez BASF Schweiz AG Schwarzwaldallee 215, CH-4002, Basel, Switzerland Prof. F. C. Krebs Department of Energy Conversion and Storage Technical University of Denmark Frederiksborgvej 399, DK-4000 Roskilde, Denmark efficiencies (PCEs) of OPV devices already surpassed the 10% milestone, [5] occupying an important position in energy infra- structure in the near future. However, the commonly used deposition method, i.e., spin-coating in inert atmosphere, is not compatible with mass-production tech- nology, such as roll-to-roll production. [6,7] Thus, the research on large-scale pro- duced OPV devices at the moment lags far behind the state-of-the-art devices. It is obvious that significant efforts have to be made when progressing from lab-scale to roll-to-roll production. [7,8] The tandem concept, in which single- junction solar cells with complementary absorption spectra are stacked together either in series or in parallel, was recently proven to simultaneously tackle the two main loss mechanisms in organic solar cells, i.e., losses due to narrow absorption of donor materials as well as thermaliza- tion losses. [9] According to previous predic- tions, tandem solar cells are supposed to achieve a theoretical PCE of 15%, [10] while the PCE of single- junction solar cells is calculated to be of 10–11% under the same conditions. [11] Recently, several research groups have reported solution-processed organic tandem solar cells with PCEs in the range of 8–10%, [12–19] higher than the performance of corresponding single-junction cells. This is a strong argu- ment for large-scale production of organic solar cells using the Adv. Energy Mater. 2014, 1400084