Abstract
The inverted organic solar cell was fabricated by using sol-gel indium-gallium-zinc-oxide (IGZO) as the electron-transport layer. The IGZO precursor solution was deposited by blade coating with simultaneous substrate heating at 120 °C from the bottom and hot wind from above. Uniform IGZO film of around 30 nm was formed after annealing at 400 °C. Using the blend of low band-gap polymer poly[(4,8-bis-(2-ethylhexyloxy)-benzo(1,2-b:4,5-b’)dithiophene)-2,6-diyl-alt- (4-(2-ethylhexanoyl)-thieno [3,4-b]thiophene-)-2-6-diyl)] (PBDTTT-C-T) and [6,6]-Phenyl C71 butyric acid methyl ester ([70]PCBM) as the active layer for the inverted organic solar cell, an efficiency of 6.2% was achieved with a blade speed of 180 mm/s for the IGZO. The efficiency of the inverted organic solar cells was found to depend on the coating speed of the IGZO films, which was attributed to the change in the concentration of surface OH groups. Compared to organic solar cells of conventional structure using PBDTTT-C-T: [70]PCBM as active layer, the inverted organic solar cells showed significant improvement in thermal stability. In addition, the chemical composition, as well as the work function of the IGZO film at the surface and inside can be tuned by the blade speed, which may find applications in other areas like thin-film transistors.
Highlights
Organic solar cell based on conjugated polymer and fullerene derivative is an attractive emerging technology due to its low-cost solution process, flexibility, and the absence of toxic materials in the device
Smooth and uniform IGZO film can be obtained from blade speed below certain values
Uniform IGZO film of several nanometers can be deposited by blade coating with substrate heating using the sol-gel method
Summary
Organic solar cell based on conjugated polymer and fullerene derivative is an attractive emerging technology due to its low-cost solution process, flexibility, and the absence of toxic materials in the device. In the inverted solar cell, zinc oxide (ZnO) is commonly used as the electron-transport layer beneath the organic active layer. In order to reduce the fabrication cost, solution process based on sol-gel method is preferred to the vacuum deposition such as sputtering. Most of the reports on the sol-gel process for ZnO in the inverted organic solar cell are based on spin coating, which cannot be scaled up and incompatible with roll-to-roll fabrication. In order to fulfill the promise of low-cost high-volume fabrication of inverted organic solar cell, both the inorganic electrontransport layer and the organic active layer have to be continuously coated in large area with high uniformity. Blade coating for organic layers in organic light-emitting diode and solar cell has been shown to give high uniformity in large areas.. For the same organic materials in the bulk heterojunction, such efficiency is similar to the results for spin-coated sol-gel ZnO. Our approach is much easier to scale up because all the layers are deposited by blade coating except for the evaporated top electrode
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