Running on starlight

10 October 2013

The team at the University of Hyogo

The team at the University of Hyogo L-R: N. Matsuo, T. Kobayashi and A. Heya

Cross-section of the proposed solar cell

Cross-section of the proposed solar cell with pn-n layer direction parallel to the light receiving surface

Researchers at the University of Hyogo, Japan, are working on using the field effect of MOS diodes to increase the amount of energy solar cells extract from sunlight.

Realising potential

Solar cells are of great interest in many applications, owing to the abundant energy potentially available in sunlight. A key factor in the viability of solar cells as power sources is their conversion efficiency, which quantifies the amount of electrical energy they can produce in relation to the amount of available solar energy.

There are many factors that reduce the amount of energy solar cells can extract for electrical power generation. Such loss mechanisms include the mismatch of the energy-band gap of the power generation layer and the photon energy of the incident solar beam, as well as the reflection of the solar beam at the light receiving surface, which is a particular problem in the UV range.  One of the most important loss mechanisms that limits conversion efficiency is the recombination of holes and electrons, excited by the light shining on the solar cell, before they can be collected at the gates to produce useful power. This mechanism accounts for around 20% of all the losses incurred in solar cells.

Recombination has been mainly addressed in two ways. The first of these is the back surface field (BSF) method, where a more heavily p-type doped layer is introduced at the back of the cell to repel electrons from the base of the cell. The second, used for field effect thin-film solar cells (TFSCs), is the formation of a p-type inversion layer in the intrinsic layer of the light receiving surface by an electric field. However, both methods are only effective at the surfaces of the power generation layer, whereas recombination also occurs throughout the bulk of the layer.

Field of effect

The researchers from the Advanced TFSC (thin film solar cell) R&D Group at the University of Hyogo have previously presented a design that uses a MOS diode to suppress recombination throughout the power generation layer. In this design, the field effect of a MOS diode at the side wall of the power generation layer causes Coulomb attraction or repulsion (depending on the polarity of the gate voltage) of the excited carriers, spatially separating them and so suppressing recombination.

In their Letter the Hyogo team examine the electrical characteristics of this concept through simulation, and the conversion efficiency is found to be improved by 1.5-1.6 times compared with a conventional TFSC structure.

The team are also looking at the effect of the orientation of the doped sub-layers of the power generation layer to the light receiving surface, as team member Prof. Naoto Matsuo explained: “Although the pn-n layer direction is perpendicular to the light receiving surface in the present manuscript, it is preferred for it to be parallel to the light receiving surface in the actual device structure. We have compared these two types of structure and found that the structure with the pn-n layer direction being parallel to the light receiving surface is preferable considering the gate effect.” This structure is also preferable from the manufacturing perspective.

“To fabricate the MOS gated-TFSC parallel to the light receiving surface, three lithography masks are necessary to form the silicon islands, p-type and n-type impurity ion-implantation areas, although the lithography step is not fitted to normal solar cell production. However, these processes are very simple, therefore they do not cost that much. The improvement of the conversion efficiency by the MOS gated-TFSC will overcome the increase of the fabrication process cost,” said Matsuo.

The results of their research into these structures so far lead the team to believe that the MOS gated-TFSC will be a useful addition to solar cell technology, as interest in ‘green’ ambient power sources for consumer electronics increases.

Further reading

This article is based on the Letter: ‘Improvement of conversion efficiency for solar cell with metal-oxide-semiconductor diode’ (new window).

A PDF version (new window) of this feature article is also available.

Journal content

Cover of Electronics Letters, volume 50, issue 08

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