A hybrid organic-inorganic three-dimensional cathode interfacial material for organic solar cells

An alcohol soluble hybrid organic-inorganic three-dimensional material 1,3,5,7,9,11,13,15-(9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-octavinylpentacyclo-octasiloxane (POSS-FN) has been synthesized and assessed as a cathode interlayer within organic solar cells consisting of a PBDT-BT:PC₆₁BM bulk heterojunction. For comparison, we also studied another two linear interfacial materials: a typical conjugated polymer poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl)-fluorene] (PFN) and an insulating polymer poly(4-N,N-dimethylamino-styene) (PStN) in the same system. The hybrid interlayer caused a significant improvement to the device power conversion efficiency by 32%, comparable to the other two interlayers. We found that there are two kinds of interfacial dipoles formation: one weak but unfavourable between the interlayer and the active layer, and the other larger, favourable and significant between the interlayer and the cathode. This latter factor maximized the built-in electric field across the interlayer-modified devices, which provides one of the major reasons for the improved performance. The thermodynamics study revealed that the driving force for the dipole formation could be ascribed to the amino groups.