Photovoltaic solar cells convert sunlight directly to electricity, providing an inexhaustible and environmentally benign energy source. In principle, nature permits us to convert up to 86.8% of the incident solar radiation into electricity, but in practice the power conversion efficiency of most solar cells fall well short of this limit. The quantum photovoltaics group performs the fundamental research required to demonstrate high efficiency solar cells and develop them into a cost effective technology.
Our present research activities include:
- High efficiency solar cells; both cell development and power prediction models.
- Nanophotonic structures for photovoltaics
- Up conversion & intermediate band solar cells
- Theory and proof of principle of other high-efficiency solar cell concepts.
Principally, it is the spectral breadth of sunlight that presents the greatest difficulty for efficient photovoltaic energy conversion. Useful power is delivered from the UV to near-IR and an efficient solar cell must collect as much of this incident light as possible. Conventional solar cells achieve this by indiscriminately absorbing photons above some threshold energy and discarding any excess energy as heat. This limits the efficiency of conventional photovoltaic convertors to 31%, the so called Shockley Queisser limit. The key to high efficiency is to either fabricate spectrally selective solar cells that preserve the energy of absorbed photons or modify the solar spectrum, so that its bandwidth is narrower and better matches a single solar cell.
Multi-junction solar cells
The most common approach to high efficiency photovoltaic power conversion is to partition the solar spectrum into separate bands and each absorbed by a cell specially tailored for that spectral band. This multi-junction approach requires careful control of the solar cell absorption bandwidth and we have pioneered an approach using quantum wells that enable us to optimally match our component junctions to the solar spectrum. Our research forms part of the NGCPV project, a joint FP7 EU-Japan project to develop solar concentrator technology. In addition, we are applying the unique properties of highly mismatched alloys (principally nitride and bismide semiconductors) to the problem of achieving low band-gap solar cells. We have developed predictive computer models for desiging multi-junction solar cells and have also applied these to predict the energy yield of solar concentrator systems from meterological data.
Multi-band Solar Cells
Multi-band solar cells require a material to be engineered in which multiple photon transitions can be supported. Our contribution to this particular cell concept has been the introductoni of a ratchet stage in the solar cell structure ensuring that optical transitions take place from a filled valance band state (VB) into an empty intermediate band state (IB). The electron transfer into a non-radiative state is critical to ensure long electron lifetime and strong subsequent absorption into the conduction band (CB). We recently showed that this approach has a fundamental efficiency advantage over a conventional intermediate band solar cell.
Nanophotonic structures for photovoltaics
Many solar cells (including most multi-junction solar cells) have absorber layers whose thickness is smaller or comparable to the wavelength of light that they absorb. This precludes the use of standard macroscopic light trapping techniques but requires nanophotonic structures to selectively scatter and direct light within the solar cell. Within the FP7-EU PRIMA project we developed simulation tools that calculate both the optical field and carrier transport within a photovotaic device and demonstrated the utility of Al nanoparticles for broadband, low-loss photocurrent enhancement.