Photovoltaic Module HIT®:

Converting Sunlight into Electricity with the World’s Highest Conversion Efficiency

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.

Fig. 1  How power is generated: 1. When hit by sunlight, electron-hole pairs are created. 2. The charge is separated toward the positive electrode and negative electrode. 3. Electricity is generated. Fig. 2  Heterojunction structure to prevent charge loss: A conventional structure consists of an anti-reflective layer, n-type diffusion layer, p-type silicon substrate and electrode in that order from the surface. With this structure, electrons are lost through the defect regions at the interface between the diffusion layer, electrode, and silicon substrate. The heterojunction structure, on the other hand, has an amorphous silicon layer between the n-type monocrystal silicon substrates to form an i-type amorphous silicon layer between the two silicon layers to reduce the loss of electrical charge at the interface. These silicon layers are sandwiched between the electrodes, and their surfaces are connected by tabs. The heterojunction structure can generate more power.

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.

Achieving the World’s Highest Efficiency of 25.6% in Silicon Photovoltaic Cells (Research and Development Level)

To achieve a high-output photovoltaic module, the Research and Development Department worked to improve the conversion efficiency of a practical-sized cell (more than 100 cm2), already having set the world record several times. Recently, by applying back-contact structure to heterojunction technology, researchers have been able to achieve the world’s highest cell conversion efficiency of 25.6% (publicized in April, 2014), garnering a lot of attention. This amazing feat broke not just Panasonic’s records for practical size cell to-date, but also the world record for even small size cells (4cm2 at 25.0%, reported by University of New South Wales, 1999), which had gone unbroken for 15 years.

Graph of change in cell conversion efficiency: The cell conversion efficiency increased gradually from 20% in 2000 to 21% in 2001, to 22% in 2006, to 23% in 2009, to 24% in 2014, and then reached 25.6% in 2014 (research levels).

Figure: Improvement of Conversion Efficiency in Panasonic’s Silicon Heterojunction Solar Cells
(cell area: more than 100cm2, research level)

Panasonic HIT® Solar Cell Achieves World's Highest Energy Conversion Efficiency*1 of 25.6%*2 at Research Level

*1 According to research by Panasonic as of April 10, 2014, for non-concentrating silicon solar cells (regardless of cell area).
*2 Result of evaluations at the National Institute of Advanced Industrial Science and Technology (AIST).

Photovoltaic Module HIT

Since development began on photovoltaic module HIT in 1997, Panasonic has endlessly pursued its goal of improved conversion efficiency. In 2014, Panasonic developed a model (VBHN250SJ31) with a module conversion efficiency of 19.5% and a maximum nominal output of 250W, touting its power generation performance. Furthermore, because photovoltaic modules must maintain their generation performance over long periods of time under the harsh conditions of the natural environment, Panasonic takes their quality and reliability very seriously. Since 2013, Panasonic has increased its module output warranty period by 100%, up to 20 years (standard type), providing consumers with not just generation performance, but high quality generation.
Panasonic’s photovoltaic module HIT has earned numerous awards, including the IEEE Corporate Innovation Award, from the Institute of Electrical and Electronics Engineers, the United States’ largest conference in the fields of electrical equipment, electronics, and data transmission.
Even in solar car racing, Panasonic’s photovoltaic module has proven its high generation performance, contributing to the twice-in-a-row world championship victories of the Tokai University Team.

Home Solar System, “Sunlight for high-quality electricity generation”

In addition, Panasonic has worked hard to contribute to a richer and more comfortable lifestyle in developing nations suffering from electrical infrastructure issues, advancing projects like introduction of photovoltaic modules with power supply containers, including storage batteries and energy management systems, to off-grid elementary schools in Java, western Indonesia.

Stand-Alone Solar Power "Container" Supports Educational Development on Remote Indonesian Island

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.