The present technology can be used to generate organic photovoltaic devices, specifically PSCs with higher open-circuit voltages.
Despite extensive efforts to improve the properties of the constituent materials and morphologies of the BJH systems, competing with the power conversion efficiency (PCE) of silicon-based solar cells remains a challenge. Fullerenes are presently the most widely used and highest performing materials in part due to their high electron affinities and low reorganization energies for electron transfer. Tailoring the electronic structure of fullerene is therefore of interest, and reactions with organometallic reagents, radicals, and transition-metal complexation have been investigated to create new fullerenes for BHJs.
The fullerene reactivity is that of an electron-deficient polyolefin, and as such a dominant functionalization approach has been to use cycloaddition reactions to modify the fullerene structure. Noteworthy examples include the synthesis of PCMB via 1,3-dipolar cycloaddition. There has recently been interest in indene-C60 acceptors because these materials give rise to ca. 110 mV reduced electron affinity and higher LUMO relative to C60. It would appear that the short distance between the C60 and the π-orbitals of the addend aromatic ring of the indene affects the LUMO energy.
The present invention expands upon the hypothesis that cofacial π-orbital interactions between C60 and an attached group can effectively raise the fullerene LUMO levels. The inventors have targeted a new functionalization method that makes use of the well-known zwitterionic AlCl3-cyclobutadiene adducts. The release of the cyclobutadiene by treatment with weak Lewis bases has been found to result in Diels-Alder reactions with alkynes to give Dewer-benzene products that have nearly right angle between two cyclobutene rings. As a result, Diels-Alder adducts of similarly generated cyclobtadiene adducts with fullerenes could give rise to strong cofacial pi-orbital intereactions.
This technology reports the synthesis of mono- and multi-adducts of tetraalkylcyclobutene and tetramethylcyclobutene to fullerenes. The electrochemical, photophysical, and thermal properties of these new fullerences have been studied and in addition their use in photovoltaic devices with poly(3-hexylthiophene) have been evaluated in comparison with PCBM. Tetramethylcyclobutene-C60 mono-, bis- and tisadduct (TMCB-Mono, TMCB-bis and TMCB-tris) all exhibited higher open-circuit voltages than that of PCMB, and TMCB-mono showed comparable power conversion efficiency (2.47%) to PCBM (2.54%) devices under the same conditions. Lastly, the π-orbital interactions between the cyclobutene and the C60 cage were probed by removing the appended double bond by epoxidation. The increased electron affinity of the cyclobutane-epoxide-C60 was measured by cyclic voltammetry and calculated using density functional theory. All of the results were consistent with the hypothesis that π-π orbital interactions are an effective means to adjust the fullerene LUMO levels. In addition, the electrochemical, photophysical, and thermal properties of the newly synthesized fullerene derivatives support the proposed relationship between the functionalization, electron affinities, and photovoltaic performance.
- More comparable power conversion efficiency to silicon-based solar cells than other fullerene functionalization approaches.
- Bicontinuous nature of phases in BHJ creates a large surface to volume ratio for efficient exciton dissociation.
- Facile and low-cost fabrication methods are compatible with large scale production.