Global Climate & Energy Project
Solar EnergyAdvanced Materials and Devices for Low-Cost and High-Performance Organic Photovoltaic Cells
Start Date: January 2006
Zhenan Bao, Department of Chemical Engineering, Michael D. McGehee, Department of Materials Science and Engineering, Stanford University
This project is dedicated to the development of novel organic materials to be used in bulk heterojunction photovoltaic devices. Their properties will be customized to increase the overall energy conversion efficiency of organic PV cells. In particular, photon absorption will be enhanced by synthesizing low-bandgap organic semiconductors with sufficient charge carrier mobility for charge extraction. Also, exciton transport will be improved by using phosphorescent materials that can generate triplet excited states after photoexcitation. Triplet states exhibit longer exciton lifetimes and thus lead to enhanced exciton diffusion and reduced geminate recombination before charge separation occurs at the donor-acceptor interface
Current commercial photovoltaic cells are dominantly based on crystalline silicon technologies, with efficiencies of about 12% for the best available devices. The cost of solar electricity is of the order of $0.4/kWh, roughly ten times higher than electricity from natural gas. Although the evolution of the solar energy market since the early 1980’s shows a ~23% reduction in the cost of solar panels with each doubling cumulative installed capacity, new photovoltaic concepts and materials are still required to make solar technologies competitive on the energy market and to foster their deployment at a large scale. Organic based solar cells have the potential to bring about a major breakthrough in reducing the cost of PV cells. In fact, organic semiconductors are relatively inexpensive and can be deposited on flexible substrates in high-throughput roll-to-roll coating machines, leading to low fabrication and installation costs and to a large variety of potential applications that are not appropriate for flat panels.
To date, the energy conversion efficiency for organic PV devices is still relatively low, with record efficiencies approaching 6%, but much progress has been made in recent years. Further fundamental investigation is required to maximize the efficiency of all key physical processes: photon absorption, exciton diffusion to the donor-acceptor interface, charge separation, and charge transport to the electrodes. This project contributes to this effort by engineering semiconductor materials that would enhance some of these processes against recombination. Low-bandgap (1-2eV) semiconductor materials will be designed to absorb near-infrared light, the low-energy portion of the solar spectrum that represents up to 50% of the energy transported by solar radiation. These materials will also be engineered to efficiently generate triplet states to improve exciton transport. Triplets often exhibit greater lifetimes than singlet states (in the range of micro- to milliseconds instead of tens of nanoseconds) because radiation from triplets is a forbidden process. Moreover, triplets undergo long distance Dexter or triplet-triplet energy transfer, allowed by π-orbital overlapping across neighboring molecules, which increases the exciton diffusion length. Finally, in the design of these materials particular care will be taken in maintaining good charge carrier mobility (>10-4cm2/Vs).
Novel phosphorescent molecules with low bandgap, suitable energy levels for triplet generation, and good charge mobility will be designed, synthesized, characterized, and tested.
Low bandgap materials will be based on fused ring conjugated structures. These molecules have relatively low bandgaps that can be further decreased by increasing the conjugation length by incorporating them into conjugated polymers or oligomers. Additionally, fused ring structures have planar molecular structures that facilitate better π- π orbital overlap between adjacent molecules and therefore allow more efficient charge transport. These molecules will be designed to be deposited by spin casting. Solubilizing groups will be added to their structure for solution processing and then thermally removed after film deposition. This will enable the stacking of multiple layers without dissolving previously deposited layers, which is important for making bilayer or multijunction cells. The design of the molecular structure will be crucial for controlling molecular packing and orientation within the layers. In particular, 2D oligomers will be considered for their potential to self-organize into mesostructures, a feature that may significantly impact the charge generation, separation, and transport.
The organic compounds synthesized in this project will be characterized optically, electrically, chemically, and structurally. Various critical parameters will be investigated, including exciton diffusion length, exciton splitting and recombination rates, charge carrier mobility, and polymer stability. The crystalline morphology and the nature of the self-organized mesostructures will be investigated using Grazing Incidence X-Ray Scattering and Near Edge X-Ray Absorption Fine Structure thanks to a collaboration with the Stanford Synchrotron Radiation Laboratory. The exciton diffusion length will be measured by photoluminescence quenching, and the exciton splitting and recombination rates will be investigated with time resolved photo-induced absorption experiments. Finally, processing parameters such as solvent choice, evaporation rates, substrate surfaces, and annealing conditions will be tuned to optimize molecular ordering and film morphology.
Throughout the project, various photovoltaic device concepts will be fabricated to test the material properties and to realize working photovoltaic devices based on these materials. The design complexity of these devices will increase with time as follows: (1) polymer/TiO2 or polymer/C60 planar bilayer cells (this design will be used to characterize the organic materials); (2) polymer/C60 blended heterojunction cells; (3) polymer/TiO2 ordered bulk heterojunction cells; (4) multijunction cells; (5) multiple stacked cells.
This investigation will advance fundamental knowledge in designing and synthesizing high-performance semiconductor materials for organic photovoltaic devices. If successful, the realization of low-bandgap materials with the capability of generating triplet exciton states will significantly improve the efficiency of this class of photovoltaic technology.