Research project: Light harvesting for high-efficiency low-cost solar cells

In the primary energy conversion reaction of photosynthesis, electrons are transferred in a series of oxidation-reduction reactions across the photosynthetic membrane. Whilst photosynthesis is optimized for the production of chemical energy, the conversion system may, under special circumstances, also produce electricity.

The photosynthetic reaction centre also shares with the solar cell a fundamental challenge: how to capture solar radiation most effectively. This problem is particularly acute in crystalline silicon solar cells (the dominant photovoltaic technology today) as silicon is a poor optical absorber. Biological organisms have developed an ingenious way how to solve this problem. It is called light harvesting, and involves the transport of excitation energy (rather than charge) to enhance the optical capture cross section of the photochemical reaction. For every chlorophyll molecule that participates in the charge separation, a hundred or more other molecules (chlorophyll but also other accessory pigments) capture the energy of sunlight via light harvesting. We are trying to employ a similar philosophy to improve the operation of solar cells.

At the fundamental level, the project aims to develop an optimum solar cell where light absorption and charge separation occur in distinct parts of the device, each possibly employing different structure and different materials. This separation of roles has the potential to bring about efficient light absorption and energy conversion within a compact device, with low material requirements and low cost. If a nanomolecular structure is used for the capture of solar radiation, light is absorbed in narrow-band electron states with little energy dissipation to phonons, and these structures will have much potential for implementing “third generation” technologies to achieve very high overall conversion efficiencies.

The ultimate structure may perhaps one day have the shape of photosensitised silicon nanowire solar cell (see figure). Our current research focuses on a more prosaic planar structure, with the challenge to produce a sub-micron photosensitised crystalline silicon solar cell with efficiency approaching similar values as for standard solar cells. The research gathers momentum and significant milestones have been passed:

An ultrathin c-Si solar (possibly the world thinnest c-Si cell) has been fabricated and characterised [ see Publications ]

• Fluorescent quenching of dyes near the silicon surface has been convincingly demonstrated [ see Publications ]
• Organic structures have been covalently attached to silicon, demonstrating good surface passivation properties (see Publications]
• Along the way we have developed a novel surface recombination technique base on the Kelvin probe [ see Publications ]