Researchers use perovskite absorbers to utilize infrared light in solar cells

Researchers from Florida State University and Georgia Tech have been working on new ways for solar cells to absorb and use infrared light, a portion of the solar spectrum that is typically unavailable for solar cell technology.

'We're working on a process to optimize the efficiency of solar cells,' said Assistant Professor of Chemistry and Biochemistry Lea Nienhaus. 'The main drive is to optimize this process for solar applications'. The team has created a new approach for solar cells to facilitate a process called photon upconversion. In photon upconversion, two low energy photons are converted into one high-energy photon that emits visible light.

Typically, these devices have used metal-organic molecules or semiconductor nanocrystals to sensitize photon upconversion, but the researchers now used a thin-film of lead-halide perovskites instead. The perovskite is coupled with a hydrocarbon called rubrene, which emits the upconverted light.

The idea behind this process is to create more efficient solar cells that can detect and utilize infrared light. Wavelengths in the infrared spectrum do not have enough energy needed to excite the electrons in a typical solar cell and therefore aren't a viable source of energy.

'That means there is a large amount of the solar spectrum that can't be absorbed by a solar cell,' Assistant Professor of Chemistry and Biochemistry Lea Nienhaus said. 'We want to turn infrared light into a wavelength that could be seen and used by a solar cell.'

To improve the device efficiency, the researchers needed to create a perovskite film that was just the right thickness. They tested films that were 20, 30, 100 and 380 nanometers thick. When the thickness was above 30 nanometers, the upconversion process became efficient under solar conditions.

'To optimize the device performance, we changed the thickness of our absorber ' the lead-halide perovskite film,' she said.

As the researchers were running the tests, they also discovered that the devices behaved in an unusual way.

Although the device was turning the infrared light into visible light, the perovskite was also reabsorbing some of the visible light created in the upconversion process.

'There's a tradeoff to using the perovskite film,' postdoctoral researcher Sarah Wieghold said. 'More visible light created in rubrene doesn't mean more light coming out of the device, which is counterintuitive.'

As a result, more detailed device engineering is required to optimize the ratio of infrared light in, versus visible light out of the device, the researchers said.