Research Projects
CAMP publication list for year 1 (.pdf)
CAMP has identified eight key research areas with opportunities for substantial improvements in organic-based molecular photovoltaics:
- (1) Quantum mechanical computation,
- (2) Synthesis and characterization of new materials,
- (3) Nanostructure engineering,
- (4) Charge carrier recombination and extraction,
- (5) Light management,
- (6) Transparent electrodes,
- (7) Third-generation concepts, and
- (8) Reliability.
- (1) Quantum mechanical computation is
very important for all aspects of molecular photovoltaics. For example, computation
techniques can be used for the design of new materials with regard to energy
level predictions, solid-state packing and optical absorption. These techniques
can also play an important role in exciton diffusion, exciton separation
at interfaces, charge transport and work function modification at interfaces.
This work will be led by Prof.
Jean-Luc Brédas (Georgia Tech) who will work closely
with our CAMP researchers in a very interdisciplinary way to address these
research topics.
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- (2) Synthesis and characterization
of new materials. Working in an interdisciplinary manner with all
other CAMP teams, our Synthesis team will prepare new molecular semi-conducting
materials for the entire Center. Key aspects in this area will be balancing
properties such as solubility, thermal/photo stability, energy levels (band
gap), absorption coefficients, and charge transport to name a few. Paying
close attention to the properties described above, this team will work
on new materials development for application in solution processed bulk
hetero-junction (BHJ), vacuum deposited multi-layer small molecule, and
dye sensitized hybrid devices. The team will be led by Profs.
Zhenan Bao, Alan Sellinger (Stanford University), Mark
Thompson (University of Southern California), and Jean
Frechet (University of California-Berkeley).
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(3)
Nanostructure Engineering. In order to study and understand how morphology influences OPV device efficiency, CAMP will use various state-of-the-art techniques. Michael
Toney (Stanford Linear Accelerator) will study blend films made by McGehee using synchrotron X-ray diffraction to determine the size and orientation of crystals, and the molecular packing within the crystals. Small angle X-ray and neutron scattering (SAXS/SANS) will be used to directly probe the nanoscale blend morphology. Coupling the crystallinity information with the 3D nanostructure morphology obtained by SAXS/SANS will allow us to correlate materials structure to exciton diffusion, charge transport, and cell performance. Coupling with Toney’s other diffraction studies will enable a better understanding of how the polymer microstructure and packing influence cell performance. Electrorheological processing will be carried in Gerry Fuller’s (Stanford University) labs to induce favorable nanostructure formation as a result of annealing the blend films in an electric field. - Alberto Salleo’s (Stanford University) group will study trap distributions and microstructural features and correlate with device performance by use of techniques such as photothermal deflection spectroscopy (PDS), constant photocurrent method (CPM), and cavity ringdown spectroscopy (CRDS) in an effort to identify the types of traps that limit transport. Peter Peumans’ (Stanford University) organic vapor phase deposition (OVPD) technique, that allows for a larger number of parameters to be controlled than traditional thermal evaporation, (substrate temperature, carrier gas flow rate, molecular partial pressure, carrier gas pressure, reactor wall temperature) will provide a wider range of morphology control. The Device Team will correlate all of this structural information with charge carrier mobility, excition diffusion lengths and device performance and then provide feedback to our Synthetic Team, which could adjust film morphology by tuning the side chain composition, molecular weight or regioregularity of the molecules or by introducing compatibilizing agents. - back to top
- (4) Charge Carrier Recombination and Extraction. For obtaining high fill factors and efficiencies, it is necessary to extract photo-generated charge carriers efficiently when the internal electric field is small. This task is challenging as the electron and hole pair that is generated when an exciton dissociates, called a geminate pair or CT exciton, is bound by a Coulomb force which may induce it to recombine. Furthermore, for small electric fields, charge carriers exist in the device for long times, enhancing non-geminate recombination. This part of the center will be involved with engineering active interfaces in an effort to reduce recombination. Modeling again will be important here to build a better understanding of the device operation. Peter Peumans has developed software for the simulation of carrier transport in nanostructured organic solar cells. The software simulates the hopping motion of charge carriers which are subject to the applied electric field, the built-in electric field and carrier-carrier interactions. Exciton and carrier migration rates developed in the quantum-chemical modeling work by Jean-Luc Bredas’ group will be used as input for the device modeling. The unique capability of this model is that it correctly takes into account the effect of the nanostructure on solar cell performance. This is achieved by tracing each electron and hole in a representative geometry such that correlations between electron and hole trajectories are correctly computed. The Peumans group will use these models and make them available to CAMP and anyone at KAUST for the accurate modeling of organic solar cells. Other important topics that will be studied here are rates of recombination and collection by transient photovoltage and photocurrent measurements, and buffer layers between donor and acceptor interfaces to prevent electron/hole recombination. This activity will be studied by Profs. Stacy Bent, Mike McGehee and Michael Grätzel - back to top
- (5)
Light Management in Organic
Solar Cells. Conventional molecular solar cells need to be up to
200 nm thick to absorb light efficiently at the edges of the absorption
spectrum. The design of such thick cells with a high quantum efficiency
and fill factor (i.e. efficient current extraction without losses) is challenging
for several reasons. First, since the built-in potential is dropped over
a large region, the electric field at the donor-acceptor interface is weak,
leading to slower carrier separation and an increased recombination likelihood.
Second, the space charge that builds up in a thick device under illumination
creates a barrier to transport and limits the current that can be extracted.
Third, since the charge carriers need to travel longer distances, recombination
is more likely. These problems will be addressed partially by engineering
of interfaces to slow down recombination and by developing high charge-carrier
mobility materials. Nonetheless, finding means to increase the optical
absorption so that thinner films can be used is an attractive optimization
path. Efforts within CAMP in this area include light concentration using
metal nanostructures, energy relay dyes, and characterization techniques
such as near field scanning electron optical microscopy (NSOM). PIs active
in this area are Profs. Shanhui
Fan, Mark Brongersma, Mike
McGehee, Michael Grätzel and Peter Peumans -
back to top - (6) Transparent electrodes. To truly revolutionize the way electricity is produced with molecular photovoltaics, it will be necessary to manufacture the cells at a cost of about $30/m2. To achieve such low cost with high throughput roll-to-roll coating machines, a new transparent conductor is clearly required. Sputtered thin films of metal oxides such as indium-tin-oxide (ITO) and Al-doped zinc-oxide (AZO) are currently too expensive because slow deposition on heated substrates is needed to obtain sufficient film quality. Moreover, ITO is unattractive because at the manufacturing volumes needed to supply a sizeable portion of the world’s energy with solar cells, the supply of indium will be a limitation. In addition, the brittleness of metal oxides is unattractive for use on plastic substrates. It is therefore highly desirable to develop a low-cost method for depositing transparent conductors based on abundant materials. Our team will tackle this problem using several approaches including carbon nanotubes, graphene, silver and zinc oxide nanowires, and combinations thereof. This team will be led by Profs. Mike McGehee, Peter Peumans, Yi Cui, Alberto Salleo and Zhenan Bao. - back to top
- (7)
Third-generation
concepts. Multi-junction cells are an effective way to remove the
fundamental tradeoff of a solar cell that limits the theoretical single
junction efficiency. It was shown that efficiencies exceeding 20% are realistic
for a set of tailored materials. Moreover, the multi-junction makes use
of an important technological advantage of organic materials: the growth
of high-quality films does not require epitaxial growth. The method is
based on the introduction of very thin metal films or highly doped pn “tunnel”
junctions between adjacent cells. The thin metal films result in arrays
of metal nanoparticles situated between two cells which establish ohmic
contacts with negligible damage to the material and high optical transparency.
We are also assessing the feasibility of utilizing multiple exciton generation.
PIs working in this team include Profs. Kelly
Gaffney, Mike McGehee, Michael
Grätzel, and Peter Peumans. -
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(8) Reliability. Making any kind of solar cell that lasts for ten years or more is a challenge since the cells must be exposed to sunlight and undergo wide temperature swings almost every day. Very little is understood regarding the fundamental degradation processes that dictate long term device efficiencies and lifetimes, prompting the US Department of Energy to consider reliability along with cost as the major obstacles to widespread solar implementation. While molecular solar cells offer the greatest hope for inexpensive solar technologies, they may be particularly susceptible to degradation. We are optimistic, however, that cell lifetimes >10 years can be demonstrated since lifetimes in a very closely related technology, organic light emitting diodes (OLED) have increased steadily to well over 50,000 hours of continuous operation (in some cases extrapolated lifetimes up to 107 hours have been measured) Furthermore, low-cost and effective encapsulation technologies exist and can be significantly improved. Within CAMP, we propose an aggressive research activity to characterize the fundamental degradation processes and to implement strategies that include synthesis of degradation-resistant molecular materials as well as encapsulation. We plan to use these capabilities as the basis to help motivate the construction of a major Solar Reliability Center at KAUST since the desert climate of Saudi Arabia is more aggressive towards materials and ideally suited for full scale solar reliability testing. Researchers active in this area are Profs. Dauskardt, McGehee, and Peumans. - back to top
