Designing Three-Dimensional Nanowire Arrays As Efficient Counter Electrodes for Quantum-Dot-Sensitized Solar Cells
Authors: Hu Jin-Song 
Published: February 2020

Quantum-dot-sensitized solar cell (QDSSC) has been considered as an alternative to new generation photovoltaics, but it still presents very low conversion efficiency. Besides the continuous effort on improving photoanodes and electrolytes, the focused investigation on charge transfer at interfaces and rational design for counter electrodes are recently receiving much attention. Rationally designing nanostructures aiming at solving the critical issues on counter electrodes would be possible to shed light on how to break through the conversion efficiency record of QDSSCs and push this field to move forward.
In this presentation, the effort in this group on designing new nanostructures as efficient counter electrodes for QDSSCs will be discussed. For example, tunnel junction arrays configured with degenerate n-type tin-doped indium oxide nanowire (ITO) core and degenerate p-type Cu2S nanocrystal shell (ITO@Cu2S) was designed and fabricated as new efficient counter electrode for QDSSCs. ITO nanowire array core provided a three dimensional conductive network. ITO core and Cu2S nanocrystal shell formed effective tunnel junctions with carrier transport path shorter than 100 nm. It was found that sheet resistance (Rh) was not only dependent on the electron conductivity of the substrate but also related to the semiconductor-electrode interface between CTO and catalysts. The high-quality tunnel junctions resulted in the considerable decrease in Rh of the device, and facilitated electron transfer from CTO to Cu2S. Moreover, the chemical inert nature of ITO made this type of counter electrode stable in liquid electrolyte with no intrinsic issue like copper dissolution in state-of-the-art Cu/Cu2S counter electrode. Compared with planar structures, the three-dimensional nanowire arrays presented higher surface area and easy accessibility of electrolyte, leading to higher catalytic activity of counter electrode as evidenced by apparently decreased charge transfer resistance. As a result, the power conversion efficiency of QDSSCs with the designed ITO@Cu2S nanowire counter electrodes increased by 84.5% and 33.5% compared to that with Au and Cu2S counter electrodes, respectively.[1] Furthermore, the influence of the interface of ITO/Cu2S on the conversion efficiency of QDSSCs was further investigated by using different synthetic route and the following post-treatment to fabricate the ITO@Cu2S nanowire counter electrodes.[2] More interestingly, the hierarchical ITO@Cu2S nanowire arrays were further fabricated and found to be able to further reduce the sheet resistance and charge transfer resistance between the electrolyte and the counter electrode. As a result, the QDSSCs with smaller series resistance, larger shunt resistance and thus the power conversion efficiency of over 6% has been achieved by using this kind of hierarchical ITO@Cu2S nanowire arrays as counter electrodes.[3] Finally, by combining a newly-developed high quality ∼4 nm Cd-, Pb-free Zn-Cu-In-Se alloyed QDs and a new three-dimensional counter electrode, we have achieved QDSSCs with a record certified efficiency of 11.6%.
References
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[2] Y. Jiang, X. Zhang, B.-B. Yu, Y.-G. Zou, W.-J. Jiang, J.-S. Hu, W.-G. Song, L.-J. Wan, ACS Appl. Mater. Interface, 2014, 6, 15448-15455.
[3] Y. Jiang, X. Zhang, B.-B. Yu, Y.-G. Zou, W.-J. Jiang, J.-S. Hu, W.-G. Song, L.-J. Wan, Nano Lett., 2015, 15, 3088–3095
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