Solar power is gaining popularity, even among other renewable energy sources which are good for the environment. To add to its popularity, it will be crucial to improve its light-to-electricity energy conversion efficiency. Panasonic’s Photovoltaic Module HIT, sporting the world’s highest conversion efficiency, is supported by technological development spanning 40 years—among the most important of which is silicon heterojunction solar cell technology.
*HIT is a registered trademark of the Panasonic Group.
How Panasonic’s Proprietary Silicon Heterojunction Solar Cells Work
Inside conventional silicon solar cells, one surface of the p-type silicon substrates, is formed from an n-type diffusion layer. However, because there are so many defects at the interface between the silicon substrate, the diffusion layer, and the electrode, some of the charge from the sunlight is lost, reducing conversion efficiency. The heterojunction solar cells developed by Panasonic contain n-type silicon substrate covered with high-quality non-doped i-type amorphous silicon. This structure prevents the loss of electrical charge, resulting in high conversion efficiency. In addition to the property of high conversion efficiency, there is very little reduction in output power when the module increases in temperature, meaning it supports high output power even during high temperatures such as summer heat, contributing to increased power production over the course of the year.
The History of Heterojunction Solar Cells
In 1980, the company known then as Sanyo Electric (now Panasonic) succeeded in engineering the world’s first amorphous silicon solar cells. However, with their low conversion efficiency of less than 10%, their uses were limited. In the latter half of the 1980’s, striving for higher efficiency, Sanyo developed solar cells with laminated amorphous silicon and thin-film poly-silicon. To evaluate the quality of those cells, Sanyo strove to develop a method to analyze solar cell performance, placing p-type amorphous silicon on top of the n-type silicon substrate. Working toward the goal of analysis, researchers inserted non-doped i-type amorphous silicon to prevent the interdiffusion of impurities, discovering a method which greatly improved junction characteristics.
This method was the use of amorphous silicon – not as an electricity-generating layer, but as a conductive passivation material. This was a technological innovation which flew in the face of the prevailing ideas in the solar cell world. Researchers presented this in 1990, at the 5th Photovoltaic Conference in Kyoto, an international photovoltaic technological conference, and gathered the attention of solar cell researchers from around the world. Then, in 1997, Photovoltaic Module HIT, containing the world’s first heterojunction solar cell, was brought to market.
The Latest Trends
In order to continue increasing conversion efficiency, Panasonic will continue to engage in research and development on photovoltaic modules, as well as the technology it uses to manufacture them. At the same time, it will continue to advance their high reliability and low cost, which relate directly to their value to consumers. In this way, Panasonic will continue to pursue and provide energy solutions for a richer and more comfortable life.