New solution processed bulk-heterojunction organic solar cells based on a triazine-bridged porphyrin dyad as electron donor

Abstract

An unsymmetrical porphyrin dyad (ZnP)-[triazine-Npip]-(ZnPCOOH) or P-tNp-P′ consisting of two zinc-metallated porphyrin units covalently linked through their peripheral aryl-amino groups to a central triazine group, to which an N-piperidine group is also attached, has been used in combination with PC₇₁BM ([6,6]-phenyl C₇₁ butyric acid methyl ester) as electron donor and acceptor, respectively, for the active layer of solution-processed bulk hetero-junction (BHJ) organic solar cells. The photophysical properties of P-tNp-P′ and PC₇₁BM blend films, as well as cyclic voltammetry measurements of the porphyrin dyad, suggest that P-tNp-P′ can effectively harvest photons and transfer electrons to PC₇₁BM. The BHJ organic solar cell based on the P-tNp-P′ : PC₇₁BM active layer blend in 1 : 1 weight ratio processed from THF resulted in a power conversion efficiency (PCE) of 2.91%. A significant improvement of the overall photovoltaic efficiency was achieved up to 4.16% when the active layer blend was processed from a 3% v/v mixture of pyridine in THF. This was ascribed to an enhancement of the short circuit current J_sc of the solar cell, which is related to the different surface morphology of the P-tNp-P′ : PC₇₁BM active layer blend upon addition of pyridine. The pyridine-modified active layer exhibits a higher degree of crystallinity, as confirmed by the X-ray diffraction pattern and AFM images of the corresponding film, which results in an increase of exciton dissociation efficiency and more balanced charge transport.