Abstract
Using ab-initio theoretical methods, we demonstrate possible enhancement of photo-conversion efficiency of an organic solar cell via intentional doping in molecular graphene-fullerene heterojunction [the hexabenzocoronene (HBC)-triethylene glycol (TEG)–C60 molecule]. Photoabsorption analysis indicates oxygen substitution into HBC leads to an extension of the spectra up to an infrared regime. A quantum-mechanical molecular dynamics simulation incorporating nonadiabatic electronic transitions reveals that a dissociated charge state (D+ and A-) in the O-doped system is more stable than the pristine case due to the presence of an effective barrier by the TEG HOMO/LUMO level. We also find that oxygen doping in HBC enhances the intermolecular carrier mobility after charge separation. On the other hand, the pristine molecule undergoes rapid recombination between donor and acceptor charges at the interface. These analyses suggest that the graphene oxidation opens a new window in the application of organic super-molecules to solar cells.
Highlights
Among various types of solar-cell structures,[1] including perovskite solar cells[2,3,4] with increasingly high efficiencies, organic and organic/inorganic hybrid photovoltaics (OPVs) have been attracting continuous attention due to their versatile properties such as flexibility, high processability, relatively simple fabrication processes and its cost effectiveness.[5,6,7] Basic structure of the OPV is a hetero-junction of electron transport layers (ETL) consisting of acceptor molecules (A) and hole transport layers (HTL) consisting of donor molecules (D)
The structure experimentally reported as a building block of the self-assembled solar cell has the elements, i.e. C12 and triethylene glycol (TEG) chains, covalently linked to the HBC core, in addition to the molecular structure in the figure, the essential properties and charge dynamics at the HTL/ETL interfaces can be extracted from the present simplified model
Effects of the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels modified by the O-doping are investigated by a nonadiabatic quantum-mechanical molecular-dynamics (NAQMD) simulation that incorporates electronic transitions through a surface-hopping approach. (Details are described in Refs. 26, 27, 30, and 34–36.) We have developed our own code to implement this theoretical method and have previously applied it to identifying atomic mechanisms of charge transfer in several organic molecules.[26,37,38]
Summary
Among various types of solar-cell structures,[1] including perovskite solar cells[2,3,4] with increasingly high efficiencies, organic and organic/inorganic hybrid photovoltaics (OPVs) have been attracting continuous attention due to their versatile properties such as flexibility, high processability, relatively simple fabrication processes and its cost effectiveness.[5,6,7] Basic structure of the OPV is a hetero-junction of electron transport layers (ETL) consisting of acceptor molecules (A) and hole transport layers (HTL) consisting of donor molecules (D). One of the major factors in determining the photo-conversion efficiency (PCE) of the OPV relies strongly on a charge-dissociation at the interface between the two layers.[8] In the last decades, much of research effort has been devoted to control the interface structures and/or carrier populations to increase the PCE.[8,9,10,11,12] Yamamoto et al.[13] have reported a successful synthesis of a self-assembled coaxial nanotube consisting of Gemini type hexabenzocoronene (HBC), as a segment of the HTL, and the fullerene (C60), as that of ETL, covalently bridged with triethylene glycol (TEG). The HBC is a light harvesting molecule that absorbs sunlight and excites an electron-hole pair (D*-A*). To increase the PCE of the OPV, it is desired to design and modify the HBC to have broader
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