Book Description
Organic photovoltaic is a promising technology for solar energy harvesting. The power conversion efficiency (PCE) of solution-processed bulk heterojunction (BHJ) cells has reached over ~10%. Fullerene and its derivatives have been the most investigated acceptor. However, fullerene derivatives have disadvantages: (i) weak absorption in visible and near-IR range, (ii) limited energy tunability. Promising alternative non-fullerene acceptors are limited, and the best efficiency achieved so far is ~5%.In this study, we used azadipyrromethene (ADP) as the building block to synthesize a series of electron acceptors. ADP derivatives are strong chromophores with strong absorption around ~ 600 nm. They are electro-active materials with two reduction peaks. Their optoelectronic properties can be tuned upon structural modifications. In this work, we synthesized a series of 3-dimensional (3D) conjugated homoleptic Zn(II) complexes of ADP dyes. The degree of conjugation in ADP was extended by installing phenylacetylene, ethynylthiophene and thiophene groups at the pyrrolic positions of the ADP core using Stille coupling. 3D structures of these molecules were synthesized by chelating with Zn(II). These new molecules showed broad intense red to near-IR absorption with onsets around 800 nm. The estimated LUMO energy level of Zn(II) complexes ranged from -3.60 to -3.85 eV. Their strong acceptor properties were demonstrated by fluorescence quenching experiments using poly(3-hexylthiophene) as the donor. These metal complexes quenched the fluorescence efficiently in both solutions and film. DFT calculations showed that all the metal complexes have distorted tetrahedral structures, with additional conjugated 'arms' extending in 3 dimensions. A unique feature of these complexes is that the two ADP ligands are p-stacked with each other, with frontier molecular orbitals delocalized over the two ligands. These complexes can therefore easily accept electrons, delocalize the negative charge over a large conjugated structure and have the potential of transporting charges in 3D, making them alternatives to fullerene derivatives as acceptors in organic solar cells, photo-detectors and other optoelectronic applications.Small internal reorganization energy is very desirable for high-performance optoelectronic materials, as it facilitates both charge separation and charge transport. DFT calculations were performed for a series of model molecules to gain better understanding on the energy level tuning, electron affinity, and the internal reorganizations of the electron transfer process. ADP-based compounds were more stable in their anionic state than cationic or neutral states and had high electron affinity, indicating their potential as n-type electron accepting material. The internal reorganization energy of ADPs were relatively low due its conjugated structure, and decreased by extending the conjugation via phenylethylene and ethylenethiophene substitutions, or by coordinating with BF2+. The largest decrease in reorganization energy was obtained when coordinating two azadipyrromethenes with zinc(II) to form a three-dimensional homoleptic zinc(II) complex, with calculated internal reorganization energies below 0.1 eV. These low reorganization energies are mainly due to the large rigid conjugated ¿ system. This work suggests that Zn(II) complexation is a novel strategy for obtaining materials that combine low internal reorganization energy with high electron affinity for the development of novel n-type optoelectronic materials.To further demonstrate their potential as electron acceptor, we made solar cells by blending the ADP-based molecules with a common electron donor, poly(3-hexylthiophene). All solar cells using Zn(II) complexes showed a photovoltaic effect, with a power conversion efficiency as high as 4.1%. Structure-property studies suggest that the 3D nature of these Zn(II) complexes prevents crystallization and promotes a favorable nanoscale morphology. The acceptor also significantly contributed to photocurrent generation by harvesting light between 600 nm and 800 nm. These results demonstrate a new paradigm to designing acceptors with tunable properties that can overcome the limitations of fullerenes.
