Design of efficient organic tandem cells: On the interplay between molecular absorption and layer sequence

We have carried out detailed optical simulations of tandem solar cells based on the following organic semiconductors: poly(3-hexylthiophene) (P3HT), poly[2,6-(4,4-bis-(2-ethylhexyl)-4H- cyclopenta[2,1-$b$⁠;3,4-$b′$]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT), 1-(3- methoxycarbonyl) propyl-1-phenyl[6,6] $C61$ $(PC60BM)$⁠, and 1-(3-methoxycarbonyl) propyl-1- phenyl[6,6] $C71$ $(PC70BM)$⁠. We demonstrate that out of the many possible combinations of the component materials, one specific combination emerges as the best to reduce the spectral overlap of the two bulk heterojunction blends and thereby to ensure an optimized short-circuit current density $(Jsc)$⁠. Furthermore, the calculations allow us to predict the maximum $Jsc$ achievable in tandem cells based on P3HT and PCPDTBT. Finally, we show that the efficient tandem cell realized and described recently by Kim et al. [Science 317, 222 (2007)] ensures balanced absorption in the top and bottoms cells.