Representation of a light management scheme in a photonic-structured perovskite solar cell, which enhances the solar-to-electricity conversion performance of the device while allowing the use of ultra-thin perovskite layers to improve the mechanical flexibility. When fully optimized, wave-optical front structures are capable of effectively collecting and trapping the sunlight in the cell layers. In this way, the photocurrent generated by the devices (hence, their conversion efficiency) is boosted in a wide spectral and incidence angular range.
Representation of a light management scheme in a photonic-structured perovskite solar cell, which enhances the solar-to-electricity conversion performance of the device while allowing the use of ultra-thin perovskite layers to improve the mechanical flexibility. When fully optimized, wave-optical front structures are capable of effectively collecting and trapping the sunlight in the cell layers. In this way, the photocurrent generated by the devices (hence, their conversion efficiency) is boosted in a wide spectral and incidence angular range.

Solar cells based on the mineral perovskite are reaching new efficiency highs and emerging as a promising photovoltaic technology. But there is room for further improvement, especially for thin-film flexible solar cells, where perovskites could find wide application. Before this can happen, however, the trade-off between thinner films of perovskite and dropping light absorption need to be addressed. Researchers from i3N/CENIMAT in Portugal have come up with a new optical strategy that allows for a thinner perovskite layer while boosting light absorption [Haque et al., Applied Materials Today 20 (2020) 100720 https://doi.org/10.1016/j.apmt.2020.100720 ].

“We propose an unprecedented optical strategy by designing wave-optical structured substrates that can significantly enhance light harvesting across the main solar spectrum, particularly in ultra-thin PSCs, which will be the key to realize high-efficiency and flexible solar cells,” say Sirazul Haque and Manuel João Mendes.

The wave-optical structures consist of thin layers of the perovskite methylammonium lead iodide (CH3NH3PbI3) on either SnO2 and Spiro-OMeTAD or ZnO and NiO, which are patterned with a hexagonal array of stretched hemispheres. The layers need to be deposited at low temperatures to be compatible with flexible polymer substrates.

“The wave-optical structures… can be straightforwardly fabricated by industrially-attractive patterning methods such as colloidal lithography (CL) – a highly scalable soft-lithography process capable of engineering with nano/micrometer resolution and high uniformity throughout large areas,” explain Haque and Mendes.

The patterned surface improves anti-reflection and light-scattering effects, enhancing light absorption in the perovskite layer and efficiency in solar cell devices by 20-25% compared with unpatterned devices. Moreover, the wave-optical structure is optimized with a 300 nm perovskite layer, as opposed to the conventional 500 nm, enabling a more flexible device.

“Besides allowing high broadband light absorption with thinner perovskite layers, the optical solution presented here can also be easily implemented at industrial scale, since it is based on photonic substrates that are micro-patterned prior to the PSCs’ deposition via, for instance, low-cost soft-lithography processes,” say Haque and Mendes.

The advantages of the approach lie in its practicality, the researchers believe. The addition of the wave-optical structure does not impinge on the fabrication of the solar cell, rather simply serving as the substrate. The approach is also promising for generic photonic platforms for other types of thin-film photovoltaic devices.

“In addition, [our approach] is an important pathway to mitigate the amount of hazardous/toxic compounds (e.g. Pb) present in the perovskite material,” they point out. “Our photonic substrates can be an extremely cost-effective approach as there is minimum material usage.”

The researchers are now integrating the wave-optical structures into reals perovskite solar cells and exploring additional ways to improve device efficiency and stability.