Physicists in the US have developed a new theoretical technique for calculating the properties of ultrathin solar cells. Their method suggests that designs that boost the amount of light absorbed by such cells could sometimes have an unwanted, negative effect on other aspects of the devices' performance. The team is currently developing the technique so it can be used within numerical simulations tools that are used to design solar cells.

Ultrathin solar cells have two key advantages than their thicker counterparts: less material is needed to build them, while the electrons and holes liberated by light do not have so far to travel, so reducing losses that occur when they recombine. These benefits are, however, offset by the fact that thin devices absorb less light than thick devices, which is why researchers are keen to use nanometre-sized structures that increase the amount of light that interacts with ultrathin cells.

These structures take advantage of near-field optical effects such as the interaction of light with surface plasmons – oscillations in the electron density on a metallic surface. Unfortunately, these near-field effects can also affect the rate at which electrons and holes recombine within the cell, which could reduce the performance of the cell.

Thermodynamics in action

While absorption in ultrathin solar cells is relatively well understood, the effect of near-field optics on electron–hole recombination is not. Now, however, Avi Niv and colleagues at the University of California, Berkeley, have developed a new way of evaluating solar-cell efficiencies that they say can be applied to extremely thin devices. The technique takes advantage of the "fluctuation-dissipation theorem", which makes the connection between a system in thermal equilibrium and the response of the system to a tiny disturbance.

In the case of a solar cell, the system at equilibrium involves a number of processes, including the absorption and emission of photons. To calculate the efficiency of a solar cell, physicists must know both the "photocurrent" – the rate at which electron–hole pairs are created – and the "recombination current", which is the rate at which electron–hole pairs recombine to make a photon. The greater the photocurrent is relative to the recombination current, the better the solar cell.

Radiating dipoles

The team treated these recombination events as fluctuations of an ensemble of radiating dipoles. This thermodynamic approach allowed them to use the fluctuation-dissipation theorem to work out the power of the recombinations in terms of thermodynamic variables, such as the temperature and chemical potential.

Niv and colleagues then used this theoretical framework to calculate the key device parameters of voltage, current and efficiency in an ultrathin solar cell. Their idealized cell comprised an ultrathin layer of the semiconductor gallium arsenide (GaAs) on a gold substrate. Light enters the solar cell through air and light that is not absorbed is reflected back from the gold for another pass through the active region of the solar cell. The team identified four possible emission channels that had to be considered – light that is emitted back into the air and light that is emitted into the gold, each of which can occur with two different polarizations.

The team calculated the emission for cell thicknesses in the range 0–300 nm and found that the emission changed as a function of thickness. In particular, the calculations predict a large peak in the emission of parallel-polarized light into the gold substrate – something that the team identify as a clear signature of near-field effects.

Pronounced dip

To examine the effect on solar-cell performance, the team then looked at a detailed balance between the rates at which photons are absorbed and emitted by the semiconductor. A calculation of the voltage created across the cell when exposed to sunlight showed a pronounced dip at 40 nm – which is to be expected because of the large emission peak at that thickness – as well as other structure related to near-field effects. By contrast, calculation of the voltage using a technique based on conventional optics does not reveal any of these features.

According to Niv, the research shows that the use of nanostructures within cells to boost their absorption of light will also have an effect on the emission of light – and both effects must be considered when determining the overall efficiency of the design.

As well as increasing the sophistication of how they used their technique to model solar cells, Niv says that the team are also working on ways that the theory could be incorporated into a numerical simulation tool that could be used to evaluate solar-cell designs.

The research is described in Physical Review Letters.