Seed Projects: Third Generation Photovoltaics

Rationally Designed Metal Complexes for Organic Photovoltaics:

Organic cells are in a class fundamentally distinct from conventional solar cells. In the latter case, excitons are generated upon light absorption. These Frenkel excitons have a large binding energy (>0.1-1 eV) and short diffusion length (~3-20 nm). To get useful work, the photoexcited donor-acceptor (DA) pair (exciton) must migrate to the interface where fields are large, and can be split into free carriers. The latter must migrate to the electrodes before recombining. In the last five years, organic thin film PV cells have progressed rapidly.
There are significant challenges at every stage in the photogeneration process :
(1) Absorption: polymers tend to absorb in a limited energy window that does not normally include the near infrared region; also their HOMO-LUMO gaps are typically too large, and they have a small absorption coefficient (a-1~100-200 nm) to efficiently absorb light within an exciton diffusion length (~3-20 nm).
(2) The mobility is low, field-dependent and strongly dependent on morphology and microstructure. Conduction along polymer chains is very different than between chains; conduction within a (1D) chain depends much more sensitively on the chain topology than in 3D, owing to traps.
(3) Charge transfer across the D/A interface can be slow. This depends on the matrix element between the constituents, which in turn depends on their relative energy levels, symmetry and wave function overlap.
(4) Polymers that tend to be the most efficient for PV tend to have poor electrochemical and thermal stability under operating conditions.
We propose to solve some of these problems by developing stable molecular and polymeric materials with wider absorption band and longer exciton diffusion length based on metal complexes. Cyclometalated Ir and Pt complexes have been widely studied for organic light emitting device (OLEDs) applications. These compounds are mostly neutral, sublimable, and phosphoresce very strongly at room temperature, demonstrating very high electrochemical and thermal stability.

Nonequilibrium Carrier Dynamics in High Efficiency Solar Cells

Several proposals currently exist that circumvent the Shockley-Queisser efficiency limit for conventional single junction solar cells. These ideas include multi-junction solar cells, photoexcitation upconversion, multi-excitation of electron-hole pairs from a single photon, secondary carrier generation due to band-to-band impact ionization by hot carriers and hot carrier extraction through selective contacts, using nanostructured materials such as quantum wells, quantum wires and quantum dots. Ross and Nozik proposed the concept of hot carrier solar cells 25 years ago as a means to circumvent the limitations imposed by the Shockley-Queisser limit in terms of both the loss of excess kinetic energy and the loss of long wavelength photons
The hot carrier solar cell consists of an ideal absorber, which corresponds to a material with a fundamental bandgap, across which electron-hole pairs are excited by photons with energies greater than EG. In the absorber, the relaxation of excess kinetic energy to the environment (i.e. the lattice) is suppressed, while the carriers themselves still interact strongly to establish a thermalized distribution, such that the electrons (and holes) are characterized by an effective temperature TH, much greater than the lattice temperature TL. In this scheme, the electrons and holes are extracted from the system before they have time to relax their excess energy, hence utilizing the total energy of the photon. To maximize efficiency, the absorber itself should be a narrow gap semiconductor. Various proposals exist for reducing the electron cooling rate. They include using nanostructured systems such as quantum wells, quantum wires, or quantum dots in direct gap polar materials, where the LO phonon emission rate may be suppressed when the intersubband spacing is less than the optical phonon energy.
Recently, another possible route was proposed: generate multiple electron-hole pairs from a single photon through the creation of secondary carriers (band-to-band impact ionization). Because this process competes with energy loss to phonons, the dynamics of carrier relaxation is of crucial importance in realizing quantum efficiencies much greater than unity. Particularly promising are nanocrystalline materials, where the reduced dimensionality of the system suppresses the dominant optical phonon relaxation mechanisms.
The modeling and simulation of such third generation concepts requires a theoretical carrier transport framework that goes well beyond present day solar device simulators based on balance equation, drift-diffusion and other quasi-stationary, local transport approaches. In this MRSEC project, we propose the modeling and simulation of third generation photovoltaic devices using direct solution of the Boltzmann Transport Equation based on Ensemble Monte Carlo simulation techniques for quantum confined systems such as quantum wires and quantum dots. This simulator includes a description of the quantized electronic states in quantum confined structures such as quantum wells and quantum dots, and the associated carrier dynamics within such systems during photoexcitation, as described in detail elsewhere. It also includes aspects of phonon dynamics through modeling of nonequilibrium phonons, and associated effects on hot carrier relaxation, as well as modification of the phonons themselves in quantum confined structures. In the proposed research, this theoretical framework will be adopted and developed to address the feasibility and potential performance of hot electron and multi-excitation solar cells.

We will address 
1) different strategies for absorber materials and selective contacts for hot photoexcited carriers in heterojunction and superlattice systems,
2) potential for multi-excitation and impact ionization by hot carriers in quantum confined systems,
3) strategies to reduce hot electron relaxation through nonequilbrium hot phonons,
4) reduced phase space for scattering in quantum confined systems,
5) phonon dynamics engineering in reduced dimensionality systems due to phonon confinement.