Design rules for semi-transparent organic tandem solar cells for window integration
Graphical abstract
Introduction
The integration of semi-transparent organic solar cells (OSCs) into facades, overhead glazing or car windows is widely considered as key application for organic photovoltaics. So far, most scientific reports on semi-transparent solar cells focus on the fabrication and the optoelectronic properties of transparent top electrodes from conductive polymers such as poly(3,4-ethylenedioxy-thiophene):polystyrenesulfonate (PEDOT:PSS) [1], [2], [3], sputtered metal oxides such as aluminum doped zinc oxide (ZnO:Al) [4] or thermally evaporated ultra-thin metal layers such as silver (Ag) [5]. Other important aspects of semi-transparent solar cells that are of utmost importance for real-life applications outside the lab, are often not appreciated – among them their transparency color perception, i.e. a color neutral appearance, a convenient color temperature and good color rendering, or the trade-off between transparency and power conversion efficiency (PCE) [4], [6]. Recently, we have demonstrated that the color rendering properties of semi-transparent solar cells can be tuned and improved by incorporating complementary absorbing dyes into the device and in particular into the polymeric electrode without affecting the device PCE [7]. Although the incorporation of dyes enables outstanding transparency color rendering properties, this approach cannot make use of the full device performance potential due to a loss of light to the dyes. Accordingly, better device performance can be achieved by employing a second complimentary absorber into the solar cell instead of a passive dye. This concept can be realized in semi-transparent tandem devices that combine two electrically active absorber materials with different absorption spectra, allowing simultaneous adjustment of the transparency color perception and enhancement in the PCE.
In this work we carry out a theoretic study of the design criteria for semi-transparent tandem solar cells with respect to the requirements for window integration. We examine the interplay of power conversion efficiency and transparency within a regime of convenient transparency color perception. Therefore, we simulate transmission spectra and current density–voltage (J–V) characteristics of tandem solar cells. Deliberately, we have chosen a parallel tandem architecture in three terminal configuration that decouples the optical and electrical properties. A tunable transparency allows for addressing different applications, e.g. integration into building facades or automotive windows.
Section snippets
Methods
For the simulation of the semi-transparent tandem solar cells and their transparency color perception we use an in-house developed software tool that employs transfer-matrix algorithms for an optical device description and drift-diffusion modelling [8] extended by a multi-trapping model for an electrical device property assessment [9], [10]. The software tool was used to describe the optoelectronic properties of organic bulk heterojunction solar cells in the past [11]. The key to the
Results and discussion
In the following, we simulate 3-terminal (parallel) tandem solar cells comprising state-of-the-art conjugated polymer absorber materials, inspired by previous experimental work [18], [19], [20], [21]. These tandem solar cells incorporate poly(3-hexylthiophene-2,5-diyl) and [6,6]-phenyl C61-butyric acid methyl ester (P3HT:PC61BM) as front (bottom) absorber layers and poly{[4,40-bis(2-ethylhexyl)dithieno(3,2-b;20,30-d)silole]-2,6-diyl-alt-(2,1,3-benzothidiazole)-4,7-diyl} and [6,6]-phenyl C71
Conclusion
We present design considerations for semi-transparent organic tandem cells to target a certain transparency while preserving a good transparency color perception, i.e. a good color rendering and convenient color temperature. Parallel three-terminal architectures allow for more degrees of freedom than series connection. We exemplified our approach using P3HT:PC61BM and PSBTBT:PC71BM absorber layers. The presented methods can be applied to other combinations of absorber materials with
Acknowledgments
We acknowledge funding by the Federal Ministry of Education and Research (BMBF) under contract 03EK3501H (project POPUP). J.M. and M.F.G.K. thank the Karlsruhe School of Optics and Photonics (KSOP) for support.
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