Compound interest

Scientists in the US are taking inspiration from the compound eyes of insects to devise a way to pack tiny solar cells together in order to develop more efficient photovoltaic devices.

In the study, a team from Stanford University has looked to the micro-lenses that form the geodesic domes of the robber fly's compound eyes. Their work points to a way to build powerful photovoltaics from the otherwise fragile photovoltaic material perovskite that would preclude from deterioration caused by heat and moisture exposure or mechanical stress. [Dauskardt et al., Energy Environ Sci (2017; DOI: 10.1039/C7EE02185B].

"Perovskites are promising, low-cost materials that convert sunlight to electricity as efficiently as conventional solar cells made of silicon, explains Reinhold Dauskardt."The problem is that perovskites are extremely unstable and mechanically fragile. They would barely survive the manufacturing process, let alone be durable long term in the environment." He points out that conventional solar panels that one might see on a rooftop are planar in design, but brittle, salt-like perovskites need a rethink if they are to become viable materials for solar energy conversion.

One answer might be seen in nature. "We were inspired by the compound eye of the fly, which consists of hundreds of tiny segmented eyes," explains Dauskardt. "It has a beautiful honeycomb shape with built-in redundancy: If you lose one segment, hundreds of others will operate. Each segment is very fragile, but it is shielded by a scaffold wall around it."

The team, which also includes Brian Watson and Adam Printz, has thus built an analogous scaffold for a compound solar cell based on cells filled with perovskite that survive fracture testing well with little loss of solar conversion efficiency. Each cell in the vast honeycomb of perovskite microcells is just 500 micrometers across. "The scaffold is made of an inexpensive epoxy resin widely used in the microelectronics industry," team member Nicholas Rolston explains. "It's resilient to mechanical stresses and thus far more resistant to fracture."

In addition to the fracture tests, the team has also carried out heat stress tests at 85 degrees Celsius and 85 percent relative humidity for six weeks. The device survived such sweltering conditions well, continuing to generate electricity at relatively high rates of efficiency. The next step is to improve how light is scattered from the scaffold into the perovskite core of each cell.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase.