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Improving Organic Solar Cell Efficiency

Understanding the optical properties of light-trapping nanostructures aids solar cell design

Seeing the Light

Organic solar cells offer the promise of low-cost, energy-generating devices that can be mass produced, yet they remain relatively low in power conversion efficiency, typically 6% – 8% for conjugated polymer/fullerene blends. Light management is key to improving efficiency, and use of nanostructures can improve light trapping in the active layer significantly.

Creating an Optical Blanket

SEM image of TiO2 nanowires

SEM image of TiO2 nanowires

Electrospun high refractive index TiO2 nanowires can act as a light-harvesting layer when applied as a top coat to solution-processed polymer bulk-heterojunction solar cells. Using a simple and scalable technique for coating, Hidetoshi Matsumoto and colleagues at the Tokyo Institute of Technology examined the relationship between the fraction of the surface covered by nanowires (coverage fraction) and the performance of the resulting organic solar cells.

Optimizing the Coverage

The TiO2 nanowires within the coating were found to be  ~135 nm in diameter on average, and ranged from 0.4 μm to 2.8 μm long. A USB-series spectrometer and tungsten halogen lamp were used to measure the relative UV-Vis reflectance spectra of the coatings, illuminating at 8° and then collecting  diffusely scattered light with an integrating sphere (the reference was a thin, uncoated slide).

Relative reflection spectra for different coverage fractions of TiO2 nanowire-coated slides.

Relative reflection spectra for different coverage fractions of TiO2 nanowire-coated slides.

Application of a coating with a 0.33 coverage fraction reduced the relative reflectivity to below 1.0 for wavelengths above ~560 nm, effectively acting as a single layer antireflection (AR) coating and trapping light within the active layer. Increasing the coverage fraction of nanowires further, however, reduced the wavelength range of this AR effect. These results allowed researchers to focus their efforts on coatings with <0.4 coverage fraction in their quest to improve light trapping efficiency.

Confirming the Boost

Current density-voltage (J-V) and external quantum efficiency measurements showed a clear enhancement effect of light intensity in the active layer due to the TiO2 nanowire coating, attributed both to light scattering within the coating and its AR effect. Reduced power conversion efficiencies for coverage fractions above 0.35 confirmed the reduced AR coating effect observed in the reflectivity study, showing the power of these measurements to predict impact on solar cell performance.

Toward a Brighter, Cleaner Future

Nanostructure coatings can boost the efficiency of solar cells by improving light absorption in the active layer. When applied via scalable coating techniques, they offer a realistic  option for low-cost organic solar devices as a source of clean, renewable energy.

reflectance-01Optical System – Reflectance


Reference: Solution-Processed Nanowire Coating for Light Management in Organic Solar Cells. Tsuboi, K.; Fukawa, T.; Konosu, Y.; Matsumoto, H.;Tanioka, A. Journal of Nanotechnology, 2012, Article ID 387586, 7 pages.