Researchers in South Korea have developed a flexible, transparent solar cell with an average visible transmittance (AVT) of 88.3%. They have also created an n-type back window layer to optimize bifacial operation.
The academics built a 20-micrometer cell for BIPV systems with a glass and colorless polyimide (CPI) substrate, an electron transport layer made of aluminum-doped ZnO (AZO), and a hydrogenated amorphous silicon (a-Si:H) absorber. Originally, they boosted the forward light flux using laser lift-off (LLO), which is claimed to move light-scattering structures to the CPI while avoiding thermal damage.
“The CPI film was separated from the glass substrate using the LLO method, in which the sacrificial a-Si:H layer was removed by laser vaporization,” the researchers explained. “Therefore, the laser power had to be optimized to minimize optical losses.”
When the LLO power was optimized to 196.5 milliwatts, the average visible transmittance (AVT) of the cell was 88.3% on the CPI substrate compared to 89.5% on the glass substrate.
“The degree of light scattering of T-CPI films can be controlled by changing the AZO etching time,” the researchers said. The short-circuit current density of the optimized cell improved from 9.11 mA/cm2 9.81 mA/cm2, while its efficiency in front incidence improved from 4.87% to 5.34%. Its efficiency to the rear was initially as low as 3.63%.
The team optimized the rear input luminous flux by implementing a dual n-type rear window layer. It “reduced optical reflection losses and facilitated electron transport at the interface, reducing carrier recombination losses for read radiation,” they said. The cell optimized for bifacial operation had an AVT of 36.9%, and its back-to-front ratio increased to about 86%.
The scientists shared their findings in the paper “Flexible and transparent thin-film light-scattering photovoltaics fabrication and optimization for bifacial operation,” published recently npj Flexible electronics.