Ligand-controlled self-organization of 1D ZnSe nanocrystals in a model bulk heterojunction solar cell system

To overcome the current limitations of organic photovoltaic cells in efficiency and durability, systematic improvements across all mechanistic aspects are essential. This paper examines the impact of incorporating rod-shaped one-dimensional (1D) ZnSe nanocrystals (NCs) with various ligands on the morphology and photovoltaic (PV) performance of a model bulk heterojunction solar cells based on poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzo-thiadia zole)] (PCDTBT) and [6,6]-phenyl C61-butyric acid methyl ester (PC 70 BM). Ligands with aromatic or aliphatic characteristics, binding through amino or thiol groups, were selected to investigate both morphological and electronic effects. We successfully demonstrate control over the self-organization of 1D ZnSe NCs within the active layer of an organic solar cell. 1D ZnSe NCs were added to the donor–acceptor solution PCDTBT/PC 70 BM, and we show that tuning the ZnSe NCs surface stabilizer determines the final 3D film composition of PV devices. The 1D ZnSe NCs were synthesized with a simple hot injection method. The original insulating charge carrier ligands used for the synthesis of 1D ZnSe nanocrystals were replaced partially by non-insulating organic compounds containing the aromatic ring and a alkyl chain. With o-phenylenediamine, a surface-segregated interfacial buffer array of ZnSe NCs formed through self-organization during the spin-coating process of the active layer solution. The morphology of the devices and the distribution of individual compounds in the active layer were studied with Atomic Force Microscopy (AFM) and Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS). These results indicated that ZnSe NCs mixed into the coating solution spontaneously accumulated on the surface of the active layer due to the low surface energy. The introduction of the 1D ZnSe NCs containing a surface modified with o-phenylenediamine in a model bulk heterojunction solar cells of ITO/PEDOT:PSS/PCDTBT:PC70BM/Al resulted in a 20 % improvement of the power conversion efficiency of the photovoltaic cells from 1.87 % to 2.25 % on average. Only in this case, a self-organized buffer array of 1D ZnSe NCs formed in a single step between the polymer and the aluminum metal electrode, likely improving solar efficiency by suppressing charge carrier recombination at the organic/metal interface.