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Microstructured porous ZnO thin film for increased light scattering and improved efficiency in inverted organic photovoltaics

Amoolya Nirmal, Aung Ko Ko Kyaw, Xiao Wei Sun, and Hilmi Volkan Demir

Open Access Open Access

Abstract

Microstructured porous zinc oxide (ZnO) thin film was developed and demonstrated as an electron selective layer for enhancing light scattering and efficiency in inverted organic photovoltaics. High degree of porosity was induced and controlled in the ZnO layer by incorporation of polyethylene glycol (PEG) organic template. Scanning electron microscopy, contact angle and absorption measurements prove that the ZnO:PEG ratio of 4:1 is optimal for the best performance of porous ZnO. Ensuring sufficient pore-filling, the use of porous ZnO leads to a marked improvement in device performance compared to non-porous ZnO, with 35% increase in current density and 30% increase in efficiency. Haze factor studies indicate that the performance improvement can be primarily attributed to the improved light scattering enabled by such a highly porous structure.

© 2014 Optical Society of America

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Figures (7)
Fig. 1
Fig. 1 Schematic representation of OPV devices with (a) a non-porous ZnO layer and (b) a highly porous ZnO layer, along with respective SEM images showing the porous and non-porous ZnO layer in the respective devices (scale bars: 20µm).

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Fig. 2
Fig. 2 Current density-voltage (J-V) characteristics of OPVs with (a) porous ZnO layer and (b)non-porous ZnO layer, with the active layer coated at different spinning speeds (reference spin-coating speed: 2000 rpm; slow spin-coating speed: 800-1000 rpm).

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Fig. 3
Fig. 3 SEM images of porous ZnO layer with the ZnO:PEG ratio of (a) 3:1 (b) 4:1 and (c) 5:1.

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Fig. 4
Fig. 4 (a) Current density-voltage (J-V) characteristics of OPVs employing porous ZnO layer with different ZnO:PEG ratios and non-porous reference cell (b) efficiency trend for the cells with different PEG ratios and non-porous reference cell extracted from 24 devices. The horizontal lines in the box denote the 25th, 50th and 75th percentile values while the error bars denote the 5th and 95th percentile values.

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Fig. 5
Fig. 5 IPCE spectra of OPVs employing ZnO layer with different ZnO:PEG ratios and non-porous reference cell.

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Fig. 6
Fig. 6 Absorption spectra of the active layer deposited on porous ZnO layer with different ZnO:PEG ratios and on non-porous ZnO layer. A slow spin speed of 800-1000 rpm is used for porous ZnO whereas spin speed of 2000 rpm is used for non-porous ZnO.

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Fig. 7
Fig. 7 (a) Total transmission (inset: diffused transmission) spectra and (b) the haze factor of the porous ZnO layer using different ZnO:PEG ratios and non-porous ZnO (reference).

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Tables (3)

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Table 1 Device parameters of porous and non-porous ZnO OPV with the active layer spin-coated at different spinning speeds (reference spin-coating speed: 2000 rpm; slow spin-coating speed: 800-1000 rpm)

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Table 2 Device parameters of the best porous ZnO OPV with different ZnO:PEG ratios and their non-porous reference device fabricated

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Table 3 Contact angle measurements of the ZnO layer in different ZnO:PEG ratios

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