Ultrathin film solar cells

For harvesting sunlight, a general minimum requirement for the absorber layer thickness can be estimated based on optical sum rules. Such calculations indicate a lower thickness limit in the range of 10-30 nm, given that the available electronic oscillator strength is utilized in an ideal sense. This is orders of magnitude below the absorber layer thicknesses employed in most thin film solar cells, demonstrating a significant potential for saving materials and resources in PV production.

If the optics permits absorption of a high fraction of the useful sunlight, advantages may also be realized in terms of the solar cell conversion efficiency and especially output voltage, energy payback time, production speed, mechanical flexibility and weight. Overall, this can increase the resource efficiency and lead to a
reduced levelized cost of electricity for solar cells, both of which are critical factors for the competitiveness of solar cells and the rate at which they can take market shares from fossil fuels, and thereby their contribution to transitioning to a more sustainable energy system.

Of course, there are many challenges in reducing the absorber layer thickness of solar cells to approach the true nanoscale (i.e. 1-100 nm). Together with the observed potential, this is indeed why the realization of ultrathin film solar cells is an exciting field for research!

Two main challenges

The two main challenges, when we compare to more ordinary thin film solar cells, are:

• Insufficient optical absorption, especially in the spectral region close to the bandgap energy, where the absorption coefficient is always weak.
• Increased surface recombination at the back of the solar cell, since this interface is now closer to the sites of electron-hole pair generation in the absorber layer.

Approaches to enhance the optical absorption include the use of resonant optical cavities, resonances in small dielectric or metallic particles, and/or the use of other types of nano- or microstructures to scatter or enhance the electromagnetic field in the absorber layer.

One focus area here is on the use of metal nanoparticles that support localized surface plasmon resonances in the visible to near infrared range. Plasmon resonances, especially when combined with an optical nanocavity, are able to effectively absorb light over an appreciable range, and can transfer the captured energy to semiconductor material in their close proximity by means of the strong near-field generated.

This approach entails many challenges in itself, such as well-controlled fabrication of metal nanostructures, compatibility with solar cell processing, and various losses introduced in the presence of metallic components. We have explored block copolymer lithography (BCPL) for the production of nanoparticle arrays, and atomic layer deposition (ALD) for the production of semiconductor layers
suitable as ultrathin absorber materials in solar cells. We make use of numerical calculations based on the finite element method (FEM) to develop designs and understanding on the nanoscale.

When it comes to the challenge of surface recombination, approaches revolve around restriction of the non-passivated contact area by means of point contacts, use of beneficial band bending and built-in fields, and carrier selective contact materials. Quantum mechanical tunneling is also of interest here, as
it can enable sufficient conduction through ultrathin passivating (dielectric) films. For this reason, and also for the dimensional scaling of the required point contact separations with the film thickness, nanoscale dimensions are again of high relevance.

We currently study, experimentally and via numerical
calculations, the interplay between various mechanisms for improved point contact designs in thin film solar cells