Ultrafast energy and electron transfer in donor-acceptor molecules for photovoltaics
Paul A. van HalEmiel PeetersB.M.W. Langeveld-VossRené A. J. JanssenGuglielmo LanzaniGiulio CerulloChristoph GadermaierM. Zavelani‐RossiS. DeSilvestriJoop KnolJan C. Hummelen
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Summary form only given. The rapid advancement in fullerene chemistry allows the covalent functionalization of C/sub 60/ with electron donors. Various C/sub 60/-based donor-acceptor dyads have been synthesized and studied to gain insight in the intramolecular photophysical processes, like energy and electron transfer. Although these dyads can serve as a model compounds for the conjugated photovoltaic cells, only a few examples have been reported with these C/sub 60/-based dyads. Apart from being well-defined model systems for photophysical characterization, the covalent linkage between donor,and acceptor in these molecular arrays provides a simple method to achieve control over the phase segregation in donor-acceptor networks. We investigate an oligo(phenylene vinylene)fullerene dyad with 4 phenyl groups (C/sub 60/-OPV/sub 4/) in solvents of different polarity using femtosecond pump-probe spectroscopy. We find that photoexcitation of the oligomer leads first to an intramolecular energy transfer to the fullerene, while an electron transfer is a secondary process, only allowed in polar solvents.Keywords:
Photoexcitation
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Electron acceptor
Electron donor
Photosensitized electron-transfer processes of fullerenes hybridized with electron donating or other electron accepting molecules have been surveyed in this review on the basis of the recent results reported mainly from our laboratories. Fullerenes act as photo-sensitizing electron acceptors with respect to a wide variety of electron donors; in addition, fullerenes in the ground state also act as good electron acceptors in the presence of light-absorbing electron donors such as porphyrins. With single-wall carbon nanotubes (SWCNTs), the photoexcited fullerenes act as electron acceptor. In the case of triple fullerene/porphyrin/SWCNT architectures, the photoexcited porphyrins act as electron donors toward the fullerene and SWCNT. These mechanisms are rationalized with the molecular orbital considerations performed for these huge supramolecules. For the confirmation of the electron transfer processes, transient absorption methods have been used, in addition to time-resolved fluorescence spectral measurements. The kinetic data obtained in solution are found to be quite useful to predict the efficiencies of photovoltaic cells.
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Organic solar cells (OSCs) have attracted considerable attention and are regarded as a promising alternative for the conversion of solar energy to electricity. Electron acceptor materials are one of the key components of OSCs. During the past decade, acceptor materials for OSCs have made important progress. Fullerenes and their derivatives are the traditional choices and the most successful acceptor materials to date, while new acceptors, such as fused-ring electron acceptors, have become a new hotspot in research on OSCs. This chapter introduces and illustrates the history, development and latest progress of acceptor materials in vacuum-deposited and solution-processed OSCs.
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Solution-processed bulk-heterojunction organic solar cells employing Ir complexes as electron donors
To explore enhancing photocurrent in organic solar cells (OSCs) via harvesting triplet excitons, two novel bicycloiridium complexes (R1 and R2) are designed and synthesized. Conventional bulk-heterojunction triplet OSCs are solution processed using R1 or R2 as sole electron donors and phenyl-C71-butyric acid methyl ester (PC71BM) as the electron acceptor. A decent short circuit current (Jsc) of 6.5 mA cm−2 is achieved though the overlap between the absorption spectrum (with ∼550 nm absorption onset) of R2 and the solar flux is relatively small. With an open circuit voltage of 0.74 V and a fill factor of 0.42, an encouraging power conversion efficiency of 2.0% is achieved in the OSCs based on R2 and PC71BM without any processing additives and post-treatments. Our preliminary result demonstrates the possibility of utilizing Ir complexes as sole electron donors in OSCs, which extends available soluble small molecules for OSCs.
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This review highlights the opportunities and challenges in stability of organic solar cells arising from the emergence of non-fullerene acceptors.
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Abstract Chlorination converts an efficient isoindigo (IID)‐based electron donor material to an effective electron acceptor material, which could pair with different donor materials. The polymer PAZ could work as the donor and achieve moderate power conversion efficiency of 3.56%, but organic solar cells based on the blend of PC 71 BM and the chlorinated analogue PAZ‐Cl fail. On the contrary, only PAZ‐Cl could be used as the acceptor material matching with typical donor polymers, such as PTB7‐Th, J52 and J52‐2Cl. Simulation based on density functional theory suggests that the main reason for this transform might be attributed to the change in the electrostatic potential of the polymers. This research paves a new way to design novel electron acceptors for organic solar cells.
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We have studied organic solar cells composed of PffBT4T-2OD as electron donor and three different electron accepting fullerenes, in order to understand the impact of different fullerenes on the morphology and efficiency of the corresponding photovoltaic devices.
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Although fullerene derivatives (e.g. PC61BM/PC71BM) are being widely used as electron acceptors in organic solar cells (OSCs), their obvious drawbacks, such as the high cost, poor absorption, limited energy levels tunability and morphological instability, have become the bottlenecks to hinder the further advancement of OSCs. Therefore, the exploration of non-fullerene electron acceptors is motivated in recent years, and the efficiencies of fullerene-free OSCs have been boosted over 13%. In this presentation, I will focus on the molecular design for highly efficient and thermally stable small molecule electron acceptors based on fused diketopyrrolopyrrole (DPP) and perylene diimide (PDI) building blocks. A new strategy of unfused-ring core is put forward to synthesize thenovel electron acceptors for OSC applications. The highly efficient and thermally stable non-fullerene organic solar cells over 11% have been fabricated by carefully designing the non-fullerene acceptors.
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