Theory
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. The modeling and simulation of third generation photovoltaic devices using direct solution of the Boltzmann Transport Equation are based on using Ensemble Monte Carlo 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:
- different strategies for absorber materials and selective contacts for hot photoexcited carriers in heterojunction and superlattice systems
- potential for multi-excitation and impact ionization by hot carriers in quantum confined systems
- strategies to reduce hot electron relaxation through nonequilbrium hot phonons
- reduced phase space for scattering in quantum confined systems
- phonon dynamics engineering in reduced dimensionality systems due to phonon confinement