Materials for Solution-processable Organic Photovoltaics (OPV)
Solution-processable organic solar cells have the potential to become a disruptive solar technology -- "solar paint"-- that is inexpensive, efficient, and mass produced. My main research interest is to understand and improve OPVs by synthesizing new materials and testing them in solar cells. The information is used to better design new materials and new fabrication processes. To further expand knowledge, we also collaborate with appropriate research groups.
Plastic solar cells have seen considerable growth over the past 15 years, and efficiencies near 10% have now been reached for single junction devices. This was possible due to the development of new materials, better control of morphology and advances in device engineering. Most of the development of new materials has been focused on optimizing the donor material when combined with fullerene-based acceptors. Much less work has been devoted to the discovery and study of new electron acceptors. However, to push the envelop further, we need to go beyond fullerene, because fullerenes have disadvantages such as poor visible to near IR light absorption and poor tuning of the energy levels. Our research aims at creating and studying novel alternative non-fullerene acceptors to enable the next generation of OPV and other optoelectronic devices.
The efficiency of OPV depends on many parameters that must be optimized simultaneously, making the development of new successfull materials very difficult. In our research, we focus on developing new acceptors that:
1) Absorb visible to near-IR light to harvest more sunlight and increase photocurrent.
2) Have tunable energy levels in order to minimize energy losses and maximize the open circuit voltage.
3) Have potential for high charge carrier mobility, a feature required for both organic solar cells, transistors and other optoelectronic devices.
4) Help create an intimate nanoscale contact between a donor and an acceptor to separate charges while ensuring chemical connectivity within each donor and each acceptor phases to collect charges at electrodes. A good morphology combined with high charge carrier mobility should translate into good fill factors, assuming other resistance losses are minimal.
Candidates for non-fullerene electron acceptors:
1) Conjugated polymers with very low bandgap and high electron affinity
Due to their intense absorption in the visible region of the spectra and their high electron affinity, azadipyrromethene (aza-DIPY) dyes are promising candidates as alternative non-fullerene electron acceptors for organic photovoltaic applications. However, their absorption band in the visible is narrow, limiting their light harvesting potential. To obtain broader absorption, we first synthesized alternating conjugated copolymers of aza-DIPY and p-phenylene ethynylene via palladium-catalyzed Sonogashira coupling. Solubility and molecular weights were tuned by varying the solubilizing groups on the aza-DIPY and co-monomer, and the optical and electrochemical properties were further modified by chelation of the aza-DIPY unit with BF2 and d10 metals. These compounds have broad absorption extending to 1000 nm and low optical bandgaps (1.24 eV to 1.54 eV). They have two reversible reductions and have high electron affinity (4.0 to 4.5 eV), making them promising n-type materials.
Gao, L.; Senevirathna, W; Sauve, G. Org. Lett. 2011, 13, 5354-5357
Gao, L.; Tang, S.; Zhu, L.; Sauve,G. Macromolecules 2012, 45, 7404-7412
2) Homoleptic metal(II) complexes as novel 3D molecular semiconductors
A novel strategy for the design and synthesis of functional materials with excellent acceptor properties is presented. The materials are based on homoleptic metal(II) complexes of azadipyrromethene (aza-DIPY) derivatives, and exhibit intense red absorption and high electron affinity. Their strong accepting properties were demonstrated by fluorescence quenching experiments using poly(3-hexylthiophene) as the donor. DFT calculations showed that the homoleptic metal(II) complexes of 2,8-di(4-tert-butylphenylacetylene)- 1,3,5,7-tetraphenylazadipyrromethene had a similar distorted tetrahedral geometry to the complexes of 1,3,5,7-tetraphenylazadipyrromethene, but with additional conjugated ‘arms’ extending in 3 dimensions (3D). A unique feature of these complexes is that the two aza-DIPY 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. These properties make them attractive alternatives to fullerene derivatives for use as acceptors in organic solar cells, photo-detectors and other optoelectronic applications.
Senevirathna, W; Sauvé, G. J. Mater. Chem. C 2013, 1, 6684-6694
Core-substituted naphthalene diimide-based materials
Core-substituted naphthalene diimides (core-substituted NDIs) were incorporated into rod-like molecules and oligomers through reaction at the imide nitrogen positions. N,N'- Di(4-bromophenyl)-2,6-di(N-alkylamino)-1,4,5,8-naphthalenetetracarboxydiimide was synthesized in only three steps, and used as a versatile platform to prepare extended struc- tures by reaction with thiophene substrates using Suzuki-coupling conditions. The opto-electronic properties of the new compounds were examined by UV/vis absorption spectroscopy, fluorescence spectroscopy, cyclic voltammetry and theoretical calculations. The imide substituents had little effect on the optical and electrochemical properties of core-substituted NDIs in solution. A bathochromic shift of the absorption was observed upon film formation, accompanied by quenching of fluorescence. These observations are consistent with increased inter-molecular interactions between core-substituted NDI moi- eties in the solid state. All compounds were tested in organic solar cells by blending with poly(3-hexylthiophene), and several showed a photovoltaic effect, demonstrating their potential as electron acceptors in organic solar cell. The best solar cell was observed for core-substituted NDI with 4-(thiophen-2-yl)phenyl imide substituents (5a), showing a power conversion efficiency of 0.57% and a large open circuit voltage of 0.87 V. This approach allows new structure–property relationship studies of non-fullerene acceptors in organic solar cells, where one can vary the imide substituent to optimize photovoltaic parameters while keeping the optical and electrochemical properties constant.
Fernando, R.; Mao, Z.; Sauvé, G. Org. Electron. 2013, 14, 1683-1692.
Synthesis of Perfluoroalkyl End-Functionalized Poly(3-hexylthiophene) and the Effect of Fluorinated End-Groups on Solar Cell Performance
A series of well-defined perfluoroalkyl end-functionalized poly(3-hexylthiophenes) (P3HT) were synthesized by Stille coupling of stannylated 2-perfluoralkylthiophene with the bromine end of P3HT. The length of the perfluoroalkyl end group was varied from −C4F13 to −C8F17. These polymers were fully characterized and tested in bulk heterojunction solar cells with phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor. Performance of the solar cells was highest for the unmodified P3HT and decreased as the length of the perfluoroalkyl end increased. The most affected device parameters were the short-circuit current density (Jsc) and series resistance, pointing to lower charge carrier mobility and poor morphology as the cause for the lower performance. While the morphology of blends did not significantly change with perfluoroalkyl end modification, analysis of blended films by energy-filtered transmission electron microscopy (EF-TEM) revealed wider P3HT domains, consistent with the perfluorinated end groups segregating to the edge or exterior of P3HT domains, causing two domains to join. This study demonstrates that the perfluoroalkyl end group can be detrimental to polymer solar cell device performance, and further work toward understanding the interface between the donor and acceptor phases is required to fully understand this effect.
Mao, Z.; Vakhshouri, K.; Jaye, C.; Fischer, D. A.; Fernando, R.; DeLongchamp, D. M.; Gomez, E. D.; Sauvé, G.Macromolecules, 2013, 46, 103-112.
Materials for printable transistors
We focus our research on solution-processable n-type conjugated molecules and polymers. Great progress has been made over the past twenty years in the development of p-type organic semiconductors, but to enable plastic electronics with low power consumption, n-type organic semiconductors are also required. However, the number of available n-type organic semiconductors is more limited. Here, we are interested in exploring different chemical structural modification to optimize charge transport properties.