Inverted polymer bulk heterojunction solar cells with ink-jet printed electron transport and active layers

• Inks of ZnO precursor with different concentrations were formulated and their jetting behavior was studied along with the effect of substrate temperature during printing.
• Printing at room temperature from an ink of viscosity 4.70 cP led to formation of good quality smooth ZnO films.
• Annealing of printed ZnO films has strong effect on the device performance with lower temperatures leading to superior device efficiencies.
• Inverted device structures fabricated with printed ZnO electron transport layer and printed P3HT:PC61BM active layer show device efficiencies of ca. 3.2%, comparable to spin coated devices.

Ink-jet printing is a potentially attractive technique for printing of components for organic electronic devices primarily due to its ability to print patterned layers and reduced ink wastage. However, the mechanism of film formation is quite complex and needs an understanding of various printing parameters on the film growth. In this manuscript, we successfully demonstrate ink-jet printing of smooth zinc oxide (ZnO) thin films with controlled thickness as electron transport layers for inverted organic solar cell devices fabricated on indium tin oxide coated glass substrates. The parameters that strongly affect the formation of a continuous ZnO thin film with controlled thickness are ink concentration and viscosity, substrate surface treatment, drop spacing, substrate temperature during printing and the annealing temperature, affected by a combination of surface energetics, surface tension of the ink and the rate of solvent evaporation. The results suggest that one can achieve a transmittance of >85% for a 45 nm thin ZnO film possessing uniform structure and morphology, fabricated using a drop spacing of 40–50 μm at an ink viscosity of 4.70 cP with substrate held at room temperature. The P3HT:PC61BM inverted organic solar cell devices fabricated using printed ZnO films as electron transporting layers exhibit an efficiency of ∼3.4–3.5%, comparable to that shown by the devices fabricated on spin coated ZnO films. Finally, the device with printed P3HT:PC61BM active layer on printed ZnO layer showed a device efficiency of ca. 3.2% suggesting that nearly completely printed devices can deliver a comparable performance to the spin coated devices.

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