High Efficiency Photovoltaics
Efficiency Limits
Existing terrestrial photovoltaic modules rarely achieve solar power conversion efficiencies in excess of 15%, yet the absolute thermodynamic efficiency limit for solar power conversion is 93.6%. At present, no known practical approach can reach this high figure, but device models exist for solar cell devices that can theoretically attain an efficiency of 86.8%. Indeed, many commercial solar thermal (hot water) systems operate at efficiencies around 70%. However, there are inherent limitations with the common photovoltaic device designs that limit the thermodynamic efficiency to no more than 31%. There are three broad approaches, illustrated below, that can overcome the limitations of the present devices.
The multiple junction approach is the standard means for high efficiency operation. Conceptually the approach is straightforward, it being a stack solar cell devices with different band-gaps, each absorbing a different portion of the solar spectrum. This approach has recently yielded a 40% efficient solar cell and is likely to break 50% in a few years time. These approaches are acceptable for manufacturing high performance solar cells suitable for use in specialist applications where cell performance is of paramount importance, but it is desirable to find a cheaper implementation for terrestrial use.
Spectrum modification is a potentially inexpensive means for achieving high-efficiency operation by adjusting the spectrum of the incident sunlight so that it can be absorbed with minimal loss by a solar cell. For example, the spectral response of a solar cell can be increased through the use of up-conversion of sub-band-gap photons. Similarly, the energy lost upon absorption of a high-energy UV photons can be stemmed by first down-converting the UV photon to an energy marginally higher than the solar cell band-gap.
Hot carrier devices attain high efficiency operation by preserving the energy of photogenerated electon-hole pairs. In conventional solar cells, the energy of the absorbed photon is transferred to an excited electron-hole pair, but much of this energy is lost as the photoexcited electrons and holes rapidly relax down to the conduction and valance band-edges. In the hot carrier solar cell, this loss is stemmed by maintaining the carrier population at a higher temperature than the amibient lattice. In principle this approach can attain a conversion efficiency of 86%.
My principle interest in this field is to demonstrate a solar cell that is capable of breaking the 31% efficiency barrier using nanostructured semiconductors. As a general approach, I use of luminescence spectroscopy as a tool to identify parasitic processes in advanced, high-efficiency solar cells.
The intermediate band solar cell provides a means to increase the limiting power conversion efficiency to 63%. Conceptually it lies between the multi-junction and hot carrier approaches described above. Although first proposed in the 1960's, it has recently become a popular, thermodynamically consistent, model for unconventional, high-efficiency solar cell designs. The increase in limiting efficiency stems entirely from the presence of a radiatively efficient but electrically isolated intermediate band, located between the conduction and valance bands. The intermediate band allows sub-band gap photons to be absorbed via a two-step process, thereby increasing the short circuit current. In the radiative limit, recombination is lower than for a standard 2-band cell with the same short circuit current, as the recombination is also a two step process. To date researchers have attempted the intermediate band cell in self-assembled InAs quantum dots, but have been frustrated by fast, non-radiative recombination in these devices. My work in this area has been establish a series of luminescence spectroscopy experiments that can be used to identify promising intermediate band materials.
The quantum well solar cell represents the most efficient nanostructured solar cell achieved to date. A series of quantum wells are incorporated into a p-i-n diode that serve to extend the absorption of the cell. This can be a useful when engineering multi-junction solar cells, as the quantum wells allow the absorption profile of each sub-cell to be adjusted independently to the lattice parameter. My PhD work resulted in the first demonstration of a highly efficient GaAsP/InGaAs strain-balanced quantum well solar cell. Recent work has mainly focused upon optimising the quantum well growth conditions and incorporating the cell into a multi-junction device. The quantum well solar cell bears some similarity with the intermediate band solar cell, but with some important differences regarding the density of confined states and in the extent to which the carrier population in the confined well states are in equilibrium with those in the barrier material. 
The concept of thermal up-conversion. High energy photons are collected using a conventional photovoltaic device, but low energy photons are used to heat a luminescent material. The anti-Stokes emission from the luminescent material serves to increase the short-circuit current in the photovoltaic device and therefore increases the efficiency of the photovoltaic system. This device is hard to achieve in practice, but represents an interesting hybrid thermal/photovoltaic device concept, capable of fundamentally improving the efficiency of a standard solar cell. Others have proposed related conceptual devices, such as a combined 
Solar cells that are used to power spacecraft and satellites degrade gradually in the space environment, due to their continuous bombardment with radiation, mainly high energy electrons and protons. Currently the InGaP/GaAs/Ge triple junction solar cell is the most efficient space solar cell, with an efficiency of 29% AM0, at the beginning of life in orbit. However, by the end of the satellite's life (typically 10-20 years) the efficiency may have dropped to 23% or lower. To avoid catastrophic panel failures before the end of a satellite's useful life, it is critical that the radiation damage mechanisms in new photovoltaic materials are understood before sending them into space. In collaboration with Sharp Corporation and the 
Work between Daido Steel, Sharp Corporation and the Toyota Technological Institute has resulted in a 28% efficient, 200Wp, 400X solar concentrator module. I am currently developing a computer model to estimate the power output from multi-junction solar concentrator systems in various climates around the world. This work is performed in collaboration with Daido Steel, Sharp Corporation, the University of Toyohashi and the University of Loughborough, U.K.