Abstract
A recently developed linear-scaling density-functional theory (LS-DFT) formalism is used to calculate optical absorption spectra of hybrids of C60 and the conjugated polymers poly(para-phenylene) (PPP) and poly(para-phenylene vinylene) (PPV). The use of a LS formalism allows calculations on large systems with realistic proportions of C60, which has been of interest for the use of such materials in photovoltaics. Two different bonding structures are tested for the hybrid PPP and for both systems additional peaks are present in the absorption spectra below the original onset of absorption. By identifying the eigenstates involved in the relevant transitions, a weighted density difference is formed, demonstrating the transfer of charge between the polymer chain and the C60, in agreement with experiment. For the hybrid PPV, no additional peaks are observed in the absorption spectrum.
Highlights
Since the discovery in 1976 that conjugated polymers could be made conducting,[1,2,3] they have been the subject of much research, both for reasons of fundamental theoretical interest and due to the promise of a wide range of exciting applications
In order to test the accuracy of the method compared to traditional plane-wave density-functional theory (DFT), we have calculated the density of states and the imaginary component of the dielectric function for a small unit cell of PPV containing 56 atoms, using ONETEP both with and without the optimization of a set of conduction non-orthogonal generalized Wannier functions (NGWFs)
Two different bonding structures were used for C60–poly( para-phenylene) (PPP), which are depicted in Fig. 4(a) and (b)
Summary
Since the discovery in 1976 that conjugated polymers could be made conducting,[1,2,3] they have been the subject of much research, both for reasons of fundamental theoretical interest and due to the promise of a wide range of exciting applications. For the purposes of this work, we are interested in the application to photovoltaic cells, where polymer based devices could have a number of advantages over the standard silicon based solar cell, in terms of cost Conjugated polymers such as poly( para-phenylene) (PPP) and poly( para-phenylene vinylene) (PPV) (whose structures are depicted in Fig. 1) have been suggested as possible candidates for use as a donor when in combination with an acceptor such as the fullerene C60.5,8 For this use, certain criteria must be met, including the occurrence of charge transfer, which has been demonstrated in fullerene–polymer mixtures.[9,10,11,12] C60–PPV photovoltaic cells have been realized in a layered structure.[13,14]
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