Polymer-based parallel tandem solar cells with a transparent ferroelectric interconnecting layer

We present a parallel polymer tandem solar cell that contains a transparent ferroelectric interconnecting layer based on polyvinylidene fluoride (PVDF) between two subcells. The interconnecting layer suppresses charge recombination, which leads to a large increase in short-circuit current and, thereby, effective parallel connection of the two subcells. The tandem cells maintained open-circuit voltages in the region between those of the subcells. An additional increase in the short-circuit current of the tandem cells was attained by introducing another ferroelectric layer to suppress charge recombination at the interface between the front subcell and a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) layer. As a result, a power conversion efficiency of 5.61% was obtained with the tandem structure PVDF/PCDTBT:PC71BM/PVDF/PTB7:PC71BM (PCDTBT is poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)], PC71BM is [6,6]-phenyl-C71-butyric acid methyl ester, and PTB7 is thieno[3,4-b]-thiophene/benzodithiophene). The tandem cell was analyzed using a parallel two-diode circuit model. The results obtained from modeling showed excellent agreement with the experimental ones.