As early as the mid-1980s, inventors were building quaint cars that ran on solar energy. They competed in races like the “World Solar Challenge” all the way across Australia. These solar cars were nothing like series-pro­duced vehicles suitable for every­day use. However, established car manu­fac­turers such as Toyota or Hyundai and start-up companies have since then adopted the concept of in­te­grating solar cells, for example into the roof of the car or the outer part of its body.

The reasons for this: “There are more and more hybrid and fully electric vehicles out on the roads. They can use the solar power di­rectly for propulsion,” explains Dr. Kaining Ding from the Jülich In­sti­tute of Energy and Climate Research (IEK-5). The current trend towards electric cars is also bringing new momentum to vehicle-integrated photovoltaics. Adding to this, “Solar modules have become so in­ex­pen­sive and efficient by now that it can pay off to use them to extend the range of electric vehicles,” says the researcher.

The German start-up Sono Motors, for instance, says it plans to mass-produce a car starting in 2023 that, with 248 solar cells built into its body, can achieve an average of 112 kilometres of additional range each week from solar energy. With long hours of sunshine in the summer, the solar cells should even be able to power the car up to 245 kilometres a week.

Using the sun more efficiently

Such figures can probably be in­crea­sed even further – with a new ge­ner­ation of solar cells that generate more electricity from sunlight per square centimetre of surface area than standard cells achieving an efficiency of around 20 per cent: Jülich photovoltaic experts de­vel­oped cells with a design known as silicon heterojunction (SHJ) and with an efficiency of 24.5 per cent. This is a German record for solar cells of this type. What makes this special: this record was achieved with cells of a size that is com­mer­cially available and not on a smaller laboratory scale, as is often the case with efficiency measurements.

SHJ solar cells differ from standard solar cells, which currently account for more than 95 per cent of the entire photovoltaic market, primarily in that they have ultra-thin layers of disordered silicon atoms sur­round­ing a wafer of crystalline silicon. On the wafer surfaces, the layers inhibit the rapid re­com­bi­nation of negative and positive charge carriers ge­ner­ated by sunlight. Recombination would lead to lower efficiency.

The efficiency of SHJ solar cells is less temperature-dependent than that of standard cells. As a result, the solar cells efficiently generate electricity even when the car roof in which they are installed heats up to 70 degrees Celsius in the blazing summer sun. On the other hand, even sub-zero temperatures are no trouble. Kaining Ding underlines another advantage with regard to use in the car: “The construction of SHJ solar cells can be exceptionally thin, which makes SHJ modules lighter and more flexible than conventional solar modules.”

Utility vehicles, trucks and motor homes offer more surface area for solar modules than passenger cars. IEK-5 is a member of a consortium that is equipping a light elec­tric utility vehicle with photovoltaic modules as part of the STREET project. A life cycle assessment by the Jülich researchers has shown that such a vehicle, in operation for at least eight years, has less of an impact on the environment and climate than a corresponding electric vehicle charged exclusively using the German power grid. “We expect that the consortium’s electrically powered utility vehicle could cover more than 30 per cent of its annual mileage from the elec­tricity generated by the photo­voltaic modules if it were used for parcel delivery in Munich, for example,” says Kaining Ding.

Flexible in colour

The Jülich researchers can alleviate potential buyers of solar electric cars of one concern: the appearance of the vehicle does not have to suffer from the solar cells. The scientists have found the correlations between the thickness of a certain layer of the SHJ solar cell and the perception of colour by the human eye. With this knowledge, solar cell and car manufacturers could ensure in the future that solar modules on cars look uniform in colour. This way, they are less conspicuous and can even be used as design elements.

Frank Frick