J. Ham, J-L Lee
Pohang University of Science and Technology (POSTECH),
Keywords: organic solar cells, transparent electrode, ITO, dielectric/metal/dielectric
Summary:As a recent development in the field of solution processable light absorbers, flexible solar cells are tremendously attractive devices with the potential for applications in power generating plants, smart windows, mobiles, and various textiles. Typically, flexible solar cells are based on flexible plastic substrates such as PET (polyethylene terephthalate), PC (poly carbonate), PES (polyether sulfone), and PEN (polyethylene naphthalate). However, these plastic substrates have critical issues of their weak stability, due to high water vapor permeation rate (101 ~ 10−1 g/m2/day at 25°C) and low temperature process (< 150 oC). Since substrates based on stainless steel and metal foil have a lot of potential due to their mechanical robustness as well as their thermal and chemical stability, top-illuminated organic solar cells (OSCs) based on these opaque substrates should be commercialized. In this work, we report the novel design of nanostructured Dielectric/Metal/Polymer (DMP) with high transmittance and excellent haze in top-illuminated OSCs, resulting in increase of efficiency by 28% compared to bottom cells. This improvement was achieved by overcoming critical problems in planar Dielectric/Metal/Dielectric (DMD) based solar cells. To design advanced structure, PDMS, known as a hybrid polymer with dielectric property, was employed instead of an inorganic dielectric layer. With the low refractive index (n = 1.45, k = 0.0001) of PDMS, a transparency insensitive to the polymer thickness up to 300 m could be achieved even in on nanostructured electrode. Consequently, incident light could pass through the PDMS layer, without reflection loss, resulting in the enhanced transmittance. Additionally, we integrated inverted hexagonal pyramid patterned PDMS with on front side of MoO3/Ag by a simple lamination technique without damages to organic active layer. The nanostructured MAP allows incident lights to be diffracted to enhance light path length in the active layer, resulting in enhanced omnidirectional optical characteristics and power conversion efficiency of 6.75 %. From the device simulation using rigorous coupled wave analysis (RCWA, Rsoft DiffractMOD) software, we inferred that the lateral distribution of light propagation due to nanostructures could increase light path length in the active PTB7:PC70BM layer. Finally, the results offer new opportunities in development of top-illuminated OSCs based on flexible metal foil or stainless steel substrates, which assist industrialization of very cost-efficient solar modules.