The global demand for energy is expected to reach 16 terawatts by 2060. Today, the extraction and use of different sources of energy is the leading cause of major environmental problems. The most promising solution is to develop new and advanced renewable sources of energy such as the light-to-charge photovoltaic (PVs) cells (Dai and Zeng 2). Lewis and Nocera’s proposal was to optimise the large amount of solar radiation the earth absorbs each hour that has the potential to power the planet for the whole year (3).
PVs are expensive to manufacture, leading to less than 5% of the global energy being generated from PVs, which prevents broader penetration into global energy markets (Forest 2). The proposed solution in academia is to price Isocyanate-modified graphene in bulk-heterojunction (BHJ) organic PVs (OPVs) with poly (3-hexylthiophene) (P3HT) (Forrest 2). According to Forrest, the PVs are designed with atoms comprised of electron donors and acceptors as well as complementary electronic orbitals combined in a photoactive layer (3). The basic concept is that the excellent electronic and optical properties facilitate charge separation and migration of the electrons when excited underpinning the development of better solar-energy harvesting technologies (Stalder, Mei and Reynolds 2). The PVs are manufactured onto flexible substrates with high carrier mobility. Chang and Wu note that to optimize the energy harvesting technology, it is necessary to solve the problems related to sheet morphology on P3HT such as physical and thermal instability at high temperatures that reduce device efficiency (7).
Lewis and Nocera conducted a detailed study to determine the intermolecular and behavioral properties of the supramolecular assembly and concluded that arranging minor molecules through programmed non-covalent interactions into well-ordered active layers could revolutionize Graphene OPV performance (6). The underpinning reasons were that
- the two dimensional unique structures formed with graphene improves the donor/ acceptor interfaces between the donor and the acceptor
- the small molecule structures that are easily varied to access a broad parameter space
- thermodynamically-driven self-assembly, rather than kinetic phase segregation, control active-layer morphology (Figure 1a).
Results of a study published by Stalder, Mei and Reynolds concluded with empirical evidence that supramolecular systems can lead to breakthrough improvements in Graphene BHJ OPV efficiencies and a fundamental understanding of how electronic properties arise in organic materials (4). Steim, Kogler and Brabec recently published evidence of how a supramolecular donor-acceptor system based on the diketopyrrolopyrrole (DPP) donor and perylene diimide (PDI) acceptor scaffolds could spontaneously assemble (Figure 1b) into highly ordered films (3). That is as a consequence of complementary hydrogen bonding, π−π stacking, and van der Waals interactions when programmed into the molecular structures of the components (Li, Zhu and Yang 23). The results consistently agreed with those of Reissig and Zimmer for proposing that donor acceptor system is ideal for the improvement of solar cell efficiency because it is an efficient tool used to study how these assemblies and electronic properties work together.
The goals of this project are to
- synthesize sheets of graphene, libraries of DPP donors and PDI acceptors, whose various substituents tune absorption, frontier molecular orbital (FMO) energies, and assembly properties
- examine assembly and charge separation in solutions
- stack graphene and create highly-ordered donor-acceptor films
- measure the mobility, exciton lifetime, and efficiency in BHJ OPVs.
The aim of the proposed research is to develop a new class of organic optoelectronic materials composed of self-assembling donors and acceptors that could be used for solar energy harvesting using the well-ordered systems to understand charge-generation processes.
Experimental Design and Approach
A study by Anthony established a new concise and versatile synthetic route for preparing libraries of DPP donors and PDI acceptors (Figure 2) (7). According to Jenekhe, Lu and Alam, the graphene sheets stack with donors and acceptors with structures defined by frontier molecular orbital energies and high absorptions rates will be prepared and varied over a large range (8). The experiment will be in accordance with the findings by Forest that will be used to establish the systematic relationships between the electronic properties of the components as well as to determine how efficiently to separate and transport the charges in solutions and films of the Graphene donor acceptor superstructures (12).
A recently published paper presented an extensive study on the solution assembly of the DPP-PDI H-bonded system. The study showed that natural systems depend on ‘emergency upon assembly’ states to create superstructures because the hierarchical structures of the complex electronic and optical properties such as the photosynthetic reaction lead to the spatial structures and photo-induced separation (Li, Zhu and Yang 2). However, larger structures can be covalently linked to form super molecular structures (Anthony 2). Examples include the synthetically formed electron rich organic donor that defines structures linked in electronic coupling systems based on the behavior and properties of frontier molecular orbitals as well as helical super molecular polymeric materials. DPP and PDI acceptor components form a co-operate acceptor assembly that are designed to achieve greater charge transfer in those superstructures. A study by Anthony showed that the entire structure is biometric defined by non-covalent bonding (3). The conclusion was that these systems form segregated stacks that are ideal for electron and hole transport. Applying the same principle to graphene, I will study by variable temperature (VT) fluorescence spectroscopy, the ability of this system to undergo charge separation into [DPP+••••PDI–•] and the graphene upon photoexcitation by both hole- and electron transfer pathways. An investigation on the charge separation of different donor-acceptor pairs by VT fluorescence spectroscopy and time-resolved electron paramagnetic resonance to determine how the relative FMO levels affect charge separation efficiencies and charge migration pathways will be done.
By preparing the graphene and the donor-acceptor films using chemical vapor deposition, the organization and electronic properties of the chemical will be investigated. Film structure, charge mobility, and OPV performance will be studied to establish quantitative relationships between the layers, solid-state packing, excited state formation, charge separation, and charge migration. To examine charge trapping and recombination, I will measure the lifetime of the excited state by femtosecond transient absorption spectroscopy. The long term goals of this project are to understand how graphene in BHJ OPV properties emerge from molecular structure as well as providing training in cutting-edge interdisciplinary science that could lead to breakthroughs in solar energy harvesting.
Anthony, John E. “Small-Molecule, Nonfullerene Acceptors for Polymer Bulk Heterojunction Organic Photovoltaics†.” Chemistry of Materials 23.3 (2010): 583- 590.Print.
Chang, Haixin, and Hongkai Wu. “Graphene-based nanocomposites: preparation, functionalization, and energy and environmental applications.” Energy & Environmental Science 6.12 (2013): 3483-3507.Print.
Dai, Jun, and Xiao Cheng Zeng. “Bilayer phosphorene: effect of stacking order on bandgap and its potential applications in thin-film solar cells.” The Journal of Physical Chemistry Letters 5.7 (2014): 1289-1293. Print.
Forrest, Stephen R. “The limits to organic photovoltaic cell efficiency.” MRS bulletin 30.01 (2005): 28-32.Print.
Jenekhe, Samson A., Liangde Lu, and Maksudul M. Alam. “New conjugated polymers with donor-acceptor architectures: Synthesis and photophysics of carbazole-quinoline and phenothiazine-quinoline copolymers and oligomers exhibiting large intramolecular charge transfer.” Macromolecules 34.21 (2001): 7315-7324. Print.
Lewis, Nathan S., and Daniel G. Nocera. “Powering the planet: Chemical challenges in solar energy utilization.” Proceedings of the National Academy of Sciences 103.43 (2006): 15729-15735.Print.
Li, Gang, Rui Zhu, and Yang Yang. “Polymer solar cells.” Nature Photonics 6.3 (2012): 153-161.Print.
Reissig, Hans-Ulrich, and Reinhold Zimmer. “Donor-acceptor-substituted cyclopropane derivatives and their application in organic synthesis.” Chemical reviews 103.4 (2003): 1151-1196.Print.
Stalder, Romain, Jianguo Mei, and John R. Reynolds. “Isoindigo-Based Donor− Acceptor Conjugated Polymers.” Macromolecules 43.20 (2010): 8348-8352. Print.
Steim, Roland, F. René Kogler, and Christoph J. Brabec. “Interface materials for organic solar cells.” Journal of Materials Chemistry 20.13 (2010): 2499- 2512.Print.