23% Crystalline Silicon Solar Cell Efficiency

Once again the record for the world’s highest energy conversion efficiency has been broken for crystalline silicon-type solar cells. SANYO Electric Co., Ltd. has achieved an efficiency of 23.0%, breaking their previous record of 22.3%, at a research level for its proprietary HIT solar photovoltaic cells. The full details can be downloaded here, but the following is an overview.

1. Improving the quality of heterojunction of HIT solar cell

The structure of the HIT solar cell allows a possible reduction in recombination loss of the electrical element (charged carrier) by surrounding the energy generation layer of single thin crystalline silicon (c-Si) with high quality ultra-thin amorphous silicon (a-Si) layers. SANYO has recently managed to improve the quality of the HIT solar cell junction through developing a technology for depositing a higher quality a-Si layer over the c-Si substrate while protecting the c-Si surface from being damaged.

The result was an increase in the open circuit voltage (Voc) from 0.725V to 0.729V.

2. Reduction of optical absorption loss

In the solar cell, sunlight hitting the surface needs guidance to the c-Si, the energy generation layer, with the smallest possible absorption loss. For the HIT solar cell, reducing optical absorption loss in the a-Si layer, which covers the front and rear surfaces of the c-Si, and the transparent conductive layer was a challenge. Absorption of short-wavelength solar radiation by the a-Si layer and that of long-wavelength solar radiation by the transparent conductive layer were the causes of the optical absorption loss. SANYO has recently developed know-how to reduce optical absorption loss in both the a-Si and transparent conductive layers. As a result, the short circuit current (Isc) improved from 39.2mA/cm² to 39.5mA/cm².

3. Reduction of resistance loss

In the solar cell, generated electric current is collected and taken out through the surface grid electrode. SANYO recently realized lower-resistance electrode material for use in the grid electrode and a higher-aspect ratio through improving printing technology, leading to a success in reduction of resistance loss when an electric current flows through the grid electrode. As a result, the fill factor (FF) was improved from 0.791 to 0.80.

Sanyo

Grätzel Cells Increase Photovoltaic Efficiency

Grätzel cells are photovoltaic devices that take advantage of the interaction of a structured semiconductor less than one nanometre in size and an organic dye that acts as a solar collector.

According to Elena Guillén, member of UPO’s Coloides y Celdas Solares Nanoestructuradas (Nanostructured Colloids and Solar Cells) Group, this dye can be either synthetic or natural and can even enable the use of chlorophyll for this type of cell.

Thus, researchers at UPO have begun a study with which they hope to increase the efficiency of these eosin or mercurochrome -based organic components by incorporating ionic salts, known as green solvents, with a view to preventing evaporation of the liquid compounds and the consequent reduction in efficiency.

Previous studies show that ionic salts are less volatile and it is this characteristic that the group headed by Professor Anta seeks to exploit. “Notwithstanding its liquid state, these types of solvents have high viscosity levels and, therefore, during the coming months we will continue our study, working on different alternatives within ionic liquids, their synthesis, etc.,” comments Elena Guillén.

The pros and cons of the new generation

Although there are already some third generation photovoltaic cells on the market (for example, for recharging mobile phones), according to the researchers their practical use is anecdotal. However, due to their properties of flexibility and variety of colors and shapes, the future of these cells lies in new market niches such as decoration or use in colored windows that not only allow light through but use this light to generate electricity.

On the other hand, apart from the rapid amortization of energy production costs -estimated in one year’s use-, there is also the low cost of the materials. “Organic materials are usually cheaper,” affirms the researcher, despite which the search continues for an alternative organic dye to the one currently used, derived from ruthenium.

“The paradox lies in the fact that if one uses these cells because their competitive edge is that they are cheaper and more readily available, and then one uses a dye based on a precious metal, what is the advantage?” points out Elena Guillén.

On the other hand, the researchers are aware that it is a relatively new technology -this type of cell was invented in 1991- that still need to be greatly developed. Furthermore, the maximum efficiency obtained in laboratory is only 11%, which is competitive but it drops when extrapolated to an industrial scale.

The main technological challenge is currently the problem of cell degradation. “If you use an organic dye, it can be degraded by the action of sunlight, with the consequent reduction in useful life compared to silicon cells. On the other hand,” the researcher highlights, “our group is working on one of the key aspect for improving cell stability – elimination of the need to use liquids that can present problems with evaporation, etc. and for which, as already mentioned, our focus is on the use of ionic salts.”