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Indium Tin Oxide-Free Tandem Polymer Solar Cells on Opaque Substrates with Top Illumination

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journal contribution
posted on 27.08.2014, 00:00 by Dhritiman Gupta, Martijn M. Wienk, René A. J. Janssen
Top-illuminated, indium tin oxide (ITO)-free, tandem polymer solar cells are fabricated on opaque substrates in an inverted device configuration. In the tandem cell, a wide band gap subcell, consisting of poly­[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) blended with [70]­PCBM is combined with a small band gap subcell consisting of a mixture of poly­[{2,5-bis­(2-hexyldecyl)-2,3,5,6-tetrahydro-3,6-dioxopyrrolo­[3,4-c]­pyrrole-1,4-diyl}-alt-{[2,2′-(1,4-phenylene)­bisthiophene]-5,5′-diyl}] (PDPPTPT) and [60]­PCBM. Compared to the more common bottom-illuminated inverted tandem polymer solar cells on transparent ITO substrates, the front and back cells must be reversed when using opaque substrates and a transparent and conductive top contact must be employed to enable top illumination. A high conductive poly­(3,4-ethylenedioxythiophene):poly­(styrenesulfonate) (PEDOT:PSS) layer in combination with Ag lines surrounding the active area as current collection electrode is used for this purpose. The tandem polymer solar cell on an opaque glass/metal substrate yields a power conversion efficiency of 6.1% when the thicknesses of the photoactive layers are balanced for optimum performance. This is similar to the equivalent inverted tandem device fabricated on a transparent glass/ITO substrate.