Fraunhofer Isee achieves 40% efficiency for indoor III-v solar cell

July 17, 2025
Emiliano Bellini

Scientists from the Fraunhofer Institute for Solar Energy Systems Isee (Fraunhofer Isee) in Germany have developed an indoor Gallium Indium phosphid (GAINP) that achieved a power conversion efficiency of more than 40%.

Gainp is an III-V semiconductor material that is often used in powerful and high-frequency electronics because of the higher electron speed compared to silicon and gallium arsenide. It has an energy band gap of 1.9 EV, which the researchers described as “ideal” for indoor applications.

Gallium-Arsenide (mesh) and other III-V-Materials-named after the groups in the periodic table to which they belong to the best known in terms of efficiency potential for solar cells. However, their high production costs have so far limited them to niche applications that feed satellites and drones. On these devices, low weight and high efficiency are more urgent concerns than the costs of energy produced.

“We have investigated how well the solar cells work with different architectures under low light conditions,” said lead researcher Malte Klitzke. “It was shown that the N-moted gannp cell performs considerably better than the P-doted cell. N-doted gainp cells generate more load carriers and produce more electricity, even under very weak light. This enabled us to achieve very high efficiency in our experiments in the conversion of weak indoor light on user in use.”

In the study “Optimization of gainp absorber design for photovoltaic conversion -efficiency indoors above 40%“Published in Applied Physics LettersThe research team explained that both P-Type and N-Type Gainp Double Heterostructure (DH) were grown on the grid on Galliumarsenide (mesh) to investigate the dynamics of the load carrier, whereby doping was performed by electrochemical capacity voltage profiling.

The cells were manufactured with homojunction and architectures with rear hetero junction, with an absorber thickness of 700 Nm and 850 Nm respectively, and were processed via photolithography and wet chemical etchings. “Ohmic metal contacts were dropped off on both wafers sides, with a full area with a background and a front schedule with parallel fingers and a bus bar for an efficient current collection,” the academics said. “Devices were separated by wet chemical mesa sets and a two-layer anti-reflection coating was applied.”

The cell was found able to achieve an efficiency of 37.5% at 100 LX and 40.9% at 1000 LX. “For lighting intensities above 700 LX, the higher-doped rear hetero junction device showed the highest efficiency, and reached 41.4% at 1000 LX,” the paper notes.

The high-quality levels were attributed by the research team to the N-type Absorber that operated with little light under circumstances, which they said that it facilitates the generation of a “considerably larger” number of excess load carriers. “The design of the rear hetero junction stopped higher surplus load carrier densities, which led to a superior filling factor and open circuit voltage compared to the gay junction,” explained it further.

The developed solar cell is intended for use in Autonomous Internet of Things (IoT) applications that work indoors without an external wired food.

A recent overview of Binnen-PV cell technologies by an international research team documents more than 250 commercial and laboratory equipment with a large area and laboratory. It includes organic, coloring sensitive and perovskiet devices, as well as crystalline and amorphous silicon, III-v semiconductor, chalcogenide and emerging lead-free alternative cells. The review also contains a discussion about applications, recent progress and strategies that are used to design more stable, highly efficient cells that work on very little light.